Process for producing surface-postcrosslinked water-absorbent polymer particles

ABSTRACT

The present invention relates to a process for producing surface-postcrosslinked water-absorbent polymer particles by coating of water-absorbent polymer particles having a content of residual monomers in the range from 0.03 to 15% by weight with at least one surface-postcrosslinker and thermal surface-postcrosslinking at temperatures in the range from 100 to 180° C.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. application Ser. No. 14/443,082, filed May15, 2015, which is the is the U.S. National Stage of PCT/EP2013/073236,filed Nov. 7, 2013, which claims the benefit of EP Patent ApplicationNo. 13181703.3, filed Aug. 26, 2013, and U.S. Provisional PatentApplication No. 61/728,839, filed Nov. 21, 2012, incorporated herein byreference in its entirety.

DESCRIPTION

The present invention relates to a process for producingsurface-postcrosslinked water-absorbent polymer particles by coating ofwater-absorbent polymer particles having a content of residual monomersin the range from 0.03 to 15% by weight with at least onesurface-postcrosslinker and thermal surface-postcrosslinking attemperatures in the range from 100 to 180° C.

The preparation of water-absorbent polymer particles is described in themonograph “Modern Superabsorbent Polymer Technology”, F. L. Buchholz andA. T. Graham, Wiley-VCH, 1998, on pages 71 to 103.

Being products which absorb aqueous solutions, water-absorbent polymerparticles are used to produce diapers, tampons, sanitary napkins andother hygiene articles, but also as water-retaining agents in marketgardening. Water-absorbent polymer particles are also referred to as“superabsorbent polymers” or “superabsorbents”.

The preparation of water-absorbent polymer particles by polymerizingdroplets of a monomer solution is described, for example, in EP 0 348180 A1, WO 96/40427 A1, U.S. Pat. No. 5,269,980, WO 2008/009580 A1, WO2008/052971 A1, WO2011/026876 A1, and WO 2011/117263 A1.

Polymerization of monomer solution droplets in a gas phase surroundingthe droplets (“dropletization polymerization”) affords roundwater-absorbent polymer particles of high mean sphericity (mSPHT). Themean sphericity is a measure of the roundness of the polymer particlesand can be determined, for example, with the Camsizer® image analysissystem (Retsch Technology GmbH; Haan; Germany).

It was an object of the present invention to provide water-absorbentpolymer particles having improved properties, i.e. water-absorbentpolymer particles having a high centrifuge retention capacity (CRC) anda high absorption under a load of 49.2 g/cm² (AUHL).

It was another object of the present invention to providewater-absorbent polymer particles that possess a high centrifugeretention capacity and which impart good liquid distribution when usedin hygiene articles.

Yet another object of the present invention is to providewater-absorbent polymer particles that allow usage reduction in hygienearticles while maintaining excellent dryness.

The object is achieved by a process for producing water-absorbentpolymer, comprising the steps forming water-absorbent polymer particlesby polymerizing a monomer solution, coating of water-absorbent polymerparticles with at least one surface-postcrosslinker and thermalsurface-postcrosslinking of the coated water-absorbent polymerparticles, wherein the content of residual monomers in thewater-absorbent polymer particles prior to the coating with thesur-face-postcrosslinker is in the range from 0.03 to 15% by weight, andthe temperature during the thermal surface-postcrosslinking is in therange from 100 to 180° C.

The present invention further provides a process for producingwater-absorbent polymer, comprising the steps forming water-absorbentpolymer particles by polymerizing a monomer solution, coating ofwater-absorbent polymer particles with at least onesurface-postcrosslinker and thermal surface-postcrosslinking of thecoated water-absorbent polymer particles, wherein the content ofresidual monomers in the water-absorbent polymer particles prior to thecoating with the surface-postcrosslinker is in the range from 0.1 to 10%by weight, the surface-postcrosslinker is an alkylene carbonate, and thetemperature during the thermal surface-postcrosslinking is in the rangefrom 100 to 180° C.

The present invention is based on the finding that the level of residualmonomers in the water-absorbent polymer particles prior to the thermalsurface-postcrosslinking, the temperature of the thermalsurface-postcrosslinking, and the surface-postcrosslinker itself have animportant impact on the properties of the formed surface-postcrosslinkedwater-absorbent polymer particles.

The result of the specific conditions according to the process of thepresent invention are water-absorbent polymer particles having a highcentrifuge retention capacity (CRC) and a high absorption under a loadof 49.2 g/cm² (AUHL). That is a surprising result. It is known that thecentrifuge retention capacity (CRC) significantly decreases duringthermal surface-postcrosslinking as proven by Ullmann's Encyclopedia ofIndustrial Chemistry, 6^(th) Ed., Vol. 35, page 84, FIG. 7. Furthersurprising is that the less reactive alkylene carbonate reacts under theinventive conditions at unusual low temperatures. Other cyclicsurface-postcrosslinkers, for example 2-oxazolidinone, show a verysimilar behaviour. According to the monograph “Modern SuperabsorbentPolymer Technology”, F. L. Buchholz and A. T. Graham, Wiley-VCH, 1998,page 98, the recommended reaction temperatures for alkylene carbonatesare in the range from 180 to 215° C.

The combination of having a high centrifuge retention capacity (CRC) anda high absorption under a load of 49.2 g/cm² (AUHL) results inwater-absorbent polymer particles having a high total liquid uptake inthe wicking absorption test.

The specific conditions result further in water-absorbent polymerparticles having a reduced pressure dependency of the characteristicswelling time in the VAUL test at high centrifuge retention capacities(CRC).

The present invention further provides surface-postcrosslinkedwater-absorbent polymer particles having a centrifuge retention capacity(CRC) from 35 to 75 g/g, an absorption under high load (AUHL) from 20 to50 g/g, a level of extractable constituents of less than 10% by weight,and a porosity from 20 to 40%.

The present invention further provides surface-postcrosslinkedwater-absorbent polymer particles having a total liquid uptake ofY>−500×ln(X)+1880wherein Y [g] is the total liquid uptake and X [g/g] is the centrifugeretention capacity, wherein the centrifuge retention capacity is atleast 25 g/g and the liquid uptake is at least 30 g.

The present invention further provides surface-postcrosslinkedwater-absorbent polymer particles having a change of characteristicswelling time of less than 0.6 and a centrifuge retention capacity of atleast 35 g/g, wherein the change of characteristic swelling time isZ<(τ_(0.5)−τ_(0.1))/τ_(0.5)wherein Z is the change of characteristic swelling time, τ_(0.1) is thecharacteristic swelling time under a pressure of 0.1 psi (6.9 g/cm²) andτ_(0.5) is the characteristic swelling time under a pressure of 0.5 psi(35.0 g/cm²).

The present invention further provides fluid-absorbent articles whichcomprise the inventive water-absorbent polymer particles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a process scheme without an external fluidized bed;

FIG. 2 illustrates a process scheme with an external fluidized bed;

FIG. 3 illustrates an arrangement of the T_outlet measurement;

FIG. 4 illustrates an arrangement of a dropletizer unit;

FIG. 5 is a longitudinal cut of a dropletizer unit;

FIG. 6 is a cross-sectional view of a dropletizer unit;

FIG. 7 is a top view of the bottom of the internal fluidized bed;

FIG. 8 illustrates the openings in the bottom of the internal fluidizedbed;

FIG. 9 is the top view of a rake stirrer for the internal fluidized bed;

FIG. 10 is the cross-sectional view of a rake stirrer for the internalfluidized bed;

FIG. 11 illustrates a process scheme for surface-postcrosslinking;

FIG. 12 illustrates a process scheme for surface-postcrosslinking andcoating;

FIG. 13 illustrates a contact dryer for surface-postcrosslinking;

FIG. 14 illustrates the apparatus used to measure Volumetric AbsorbencyUnder Load (VAUL);

FIG. 15 illustrates the apparatus used to measure Wicking absorption;and

FIG. 16 is a graph of CRC (g/g) vs. Total liquid uptake (g) forwater-absorbent polymer particles.

DETAILED DESCRIPTION OF THE INVENTION

The water-absorbent polymer particles are prepared by a process,comprising the steps forming water-absorbent polymer particles bypolymerizing a monomer solution, comprising

-   a) at least one ethylenically unsaturated monomer which bears acid    groups and may be at least partly neutralized,-   b) optionally one or more crosslinker,-   c) at least one initiator,-   d) optionally one or more ethylenically unsaturated monomers    copolymerizable with the monomers mentioned under a),-   e) optionally one or more water-soluble polymers, and-   f) water,    coating of water-absorbent polymer particles with at least one    surface-postcrosslinker and thermal surface-postcrosslinking of the    coated water-absorbent polymer particles, wherein the content of    residual monomers in the water-absorbent polymer particles prior to    the coating with the surface-postcrosslinker is in the range from    0.03 to 15% by weight, the surface-postcrosslinker is an alkylene    carbonate, and the temperature during the thermal    surface-postcrosslinking is in the range from 100 to 180° C.

The water-absorbent polymer particles are typically insoluble butswellable in water.

The monomers a) are preferably water-soluble, i.e. the solubility inwater at 23° C. is typically at least 1 g/100 g of water, preferably atleast 5 g/100 g of water, more preferably at least 25 g/100 g of water,most preferably at least 35 g/100 g of water.

Suitable monomers a) are, for example, ethylenically unsaturatedcarboxylic acids such as acrylic acid, methacrylic acid, maleic acid,and itaconic acid. Particularly preferred monomers are acrylic acid andmethacrylic acid. Very particular preference is given to acrylic acid.

Further suitable monomers a) are, for example, ethylenically unsaturatedsulfonic acids such as vinylsulfonic acid, styrenesulfonic acid and2-acrylamido-2-methylpropanesulfonic acid (AMPS).

Impurities may have a strong impact on the polymerization. Preference isgiven to especially purified monomers a). Useful purification methodsare disclosed in WO 2002/055469 A1, WO 2003/078378 A1 and WO 2004/035514A1. A suitable monomer a) is according to WO 2004/035514 A1 purifiedacrylic acid having 99.8460% by weight of acrylic acid, 0.0950% byweight of acetic acid, 0.0332% by weight of water, 0.0203 by weight ofpropionic acid, 0.0001% by weight of furfurals, 0.0001% by weight ofmaleic anhydride, 0.0003% by weight of diacrylic acid and 0.0050% byweight of hydroquinone monomethyl ether.

Polymerized diacrylic acid is a source for residual monomers due tothermal decomposition. If the temperatures during the process are low,the concentration of diacrylic acid is no more critical and acrylicacids having higher concentrations of diacrylic acid, i.e. 500 to 10,000ppm, can be used for the inventive process.

The content of acrylic acid and/or salts thereof in the total amount ofmonomers a) is preferably at least 50 mol %, more preferably at least 90mol %, most preferably at least 95 mol %.

The acid groups of the monomers a) are typically partly neutralized inthe range of 0 to 100 mol %, preferably to an extent of from 25 to 85mol %, preferentially to an extent of from 50 to 80 mol %, morepreferably from 60 to 75 mol %, for which the customary neutralizingagents can be used, preferably alkali metal hydroxides, alkali metaloxides, alkali metal carbonates or alkali metal hydrogen carbonates, andmixtures thereof. Instead of alkali metal salts, it is also possible touse ammonia or organic amines, for example, triethanolamine. It is alsopossible to use oxides, carbonates, hydrogencarbonates and hydroxides ofmagnesium, calcium, strontium, zinc or aluminum as powders, slurries orsolutions and mixtures of any of the above neutralization agents.Example for a mixture is a solution of sodiumaluminate. Sodium andpotassium are particularly preferred as alkali metals, but veryparticular preference is given to sodium hydroxide, sodium carbonate orsodium hydrogen carbonate, and mixtures thereof. Typically, theneutralization is achieved by mixing in the neutralizing agent as anaqueous solution, as a melt or preferably also as a solid. For example,sodium hydroxide with water content significantly below 50% by weightmay be present as a waxy material having a melting point above 23° C. Inthis case, metered addition as piece material or melt at elevatedtemperature is possible.

Optionally, it is possible to add to the monomer solution, or tostarting materials thereof, one or more chelating agents for maskingmetal ions, for example iron, for the purpose of stabilization. Suitablechelating agents are, for example, alkali metal citrates, citric acid,alkali metal tartrates, alkali metal lactates and glycolates,pentasodium triphosphate, ethylenediamine tetraacetate, nitrilotriaceticacid, and all chelating agents known under the Trilon® name, for exampleTrilon® C (pentasodium diethylenetriaminepentaacetate), Trilon® D(trisodium (hydroxyethyl)-ethylenediaminetriacetate), and Trilon® M(methylglycinediacetic acid).

The monomers a) comprise typically polymerization inhibitors, preferablyhydroquinone monoethers, as inhibitor for storage.

The monomer solution comprises preferably up to 250 ppm by weight, morepreferably not more than 130 ppm by weight, most preferably not morethan 70 ppm by weight, preferably not less than 10 ppm by weight, morepreferably not less than 30 ppm by weight and especially about 50 ppm byweight of hydroquinone monoether, based in each case on acrylic acid,with acrylic acid salts being counted as acrylic acid. For example, themonomer solution can be prepared using acrylic acid having appropriatehydroquinone monoether content. The hydroquinone monoethers may,however, also be removed from the monomer solution by absorption, forexample on activated carbon.

Preferred hydroquinone monoethers are hydroquinone monomethyl ether(MEHQ) and/or alpha-tocopherol (vitamin E).

Suitable crosslinkers b) are compounds having at least two groupssuitable for crosslinking. Such groups are, for example, ethylenicallyunsaturated groups which can be polymerized by a free-radical mechanisminto the polymer chain and functional groups which can form covalentbonds with the acid groups of monomer a). In addition, polyvalent metalions which can form coordinate bond with at least two acid groups ofmonomer a) are also suitable crosslinkers b).

The crosslinkers b) are preferably compounds having at least twofree-radically polymerizable groups which can be polymerized by afree-radical mechanism into the polymer network. Suitable crosslinkersb) are, for example, ethylene glycol dimethacrylate, diethylene glycoldiacrylate, polyethylene glycol diacrylate, allyl methacrylate,trimethylolpropane triacrylate, triallylamine, tetraallylammoniumchloride, tetraallyloxyethane, as described in EP 0 530 438 A1, di- andtri-acrylates, as described in EP 0 547 847 A1, EP 0 559 476 A1, EP 0632 068 A1, WO 93/21237 A1, WO 2003/104299 A1, WO 2003/104300 A1, WO2003/104301 A1 and in DE 103 31 450 A1, mixed acrylates which, as wellas acrylate groups, comprise further ethylenically unsaturated groups,as described in DE 103 314 56 A1 and DE 103 55 401 A1, or crosslinkermixtures, as described, for example, in DE 195 43 368 A1, DE 196 46 484A1, WO 90/15830 A1 and WO 2002/32962 A2.

Suitable crosslinkers b) are in particular pentaerythritol triallylether, tetraallyloxyethane, polyethyleneglycole diallylethers (based onpolyethylene glycole having a molecular weight between 400 and 20000g/mol), N,N′-methylenebisacrylamide, 15-tuply ethoxylatedtrimethylolpropane, polyethylene glycol diacrylate, trimethylolpropanetriacrylate and triallylamine.

Very particularly preferred crosslinkers b) are the polyethoxylatedand/or -propoxylated glycerols which have been esterified with acrylicacid or methacrylic acid to give di- or triacrylates, as described, forexample in WO 2003/104301 A1. Di- and/or triacrylates of 3- to 18-tuplyethoxylated glycerol are particularly advantageous. Very particularpreference is given to di- or triacrylates of 1- to 5-tuply ethoxylatedand/or propoxylated glycerol. Most preferred are the triacrylates of 3-to 5-tuply ethoxylated and/or propoxylated glycerol and especially thetriacrylate of 3-tuply ethoxylated glycerol.

The amount of crosslinker b) is preferably from 0.0001 to 0.6% byweight, more preferably from 0.001 to 0.2% by weight, most preferablyfrom 0.01 to 0.06% by weight, based in each case on monomer a). Onincreasing the amount of crosslinker b) the centrifuge retentioncapacity (CRC) decreases and the absorption under a pressure of 21.0g/cm² (AUL) passes through a maximum.

The surface-postcrosslinked polymer particles of the present inventionsurprisingly require very little or even no cross-linker during thepolymerization step. So, in one particularly preferred embodiment of thepresent invention no crosslinker b) is used.

The initiators c) used may be all compounds which disintegrate into freeradicals under the polymerization conditions, for example peroxides,hydroperoxides, hydrogen peroxide, persulfates, azo compounds and redoxinitiators. Preference is given to the use of water-soluble initiators.In some cases, it is advantageous to use mixtures of various initiators,for example mixtures of hydrogen peroxide and sodium or potassiumperoxodisulfate. Mixtures of hydrogen peroxide and sodiumperoxodisulfate can be used in any proportion.

Particularly preferred initiators c) are azo initiators such as2,2′-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride and2,2′-azobis[2-(5-methyl-2-imidazolin-2-yl)propane] dihydrochloride,2,2′-azobis(2-amidinopropane) dihydrochloride,4,4′-azobis(4-cyanopentanoic acid), 4,4′-azobis(4-cyanopentanoic acid)sodium salt, 2,2′-azobis[2-methyl-N-(2-hydroxyethyl)propionamide], andphotoinitiators such as 2-hydroxy-2-methylpropiophenone and1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-1-propan-1-one, redoxinitiators such as sodium persulfate/hydroxymethylsulfinic acid,ammonium peroxodisulfate/hydroxymethylsulfinic acid, hydrogenperoxide/hydroxymethylsulfinic acid, sodium persulfate/ascorbic acid,ammonium peroxodisulfate/ascorbic acid and hydrogen peroxide/ascorbicacid, photoinitiators such as1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-1-propan-1-one, andmixtures thereof. The reducing component used is, however, preferably amixture of the sodium salt of 2-hydroxy-2-sulfinatoacetic acid, thedisodium salt of 2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite.Such mixtures are obtainable as Bruggolite® FF6 and Bruggolite® FF7(Brüggemann Chemicals; Heilbronn; Germany). Of course it is alsopossible within the scope of the present invention to use the purifiedsalts or acids of 2-hydroxy-2-sulfinatoacetic acid and2-hydroxy-2-sulfonatoacetic acid—the latter being available as sodiumsalt under the trade name Blancolen® (Brüggemann Chemicals; Heilbronn;Germany).

The initiators are used in customary amounts, for example in amounts offrom 0.001 to 5% by weight, preferably from 0.01 to 2% by weight, mostpreferably from 0.05 to 0.5% by weight, based on the monomers a).

Examples of ethylenically unsaturated monomers d) which arecopolymerizable with the monomers a) are acrylamide, methacrylamide,hydroxyethyl acrylate, hydroxyethyl methacrylate, dimethylaminoethylacrylate, dimethylaminoethyl methacrylate, dimethylaminopropyl acrylateand diethylaminopropyl methacrylate.

Useful water-soluble polymers e) include polyvinyl alcohol, modifiedpolyvinyl alcohol comprising acidic side groups for example Poval® K(Kuraray Europe GmbH; Frankfurt; Germany), polyvinylpyrrolidone, starch,starch derivatives, modified cellulose such as methylcellulose,carboxymethylcellulose or hydroxyethylcellulose, gelatin, polyglycols orpolyacrylic acids, polyesters and polyamides, polylactic acid,polyglycolic acid, co-polylactic-polyglycolic acid, polyvinylamine,polyallylamine, water soluble copolymers of acrylic acid and maleic acidavailable as Sokalan® (BASF SE; Ludwigshafen; Germany), preferablystarch, starch derivatives and modified cellulose.

For optimal action, the preferred polymerization inhibitors requiredissolved oxygen. Therefore, the monomer solution can be freed ofdissolved oxygen before the polymerization by inertization, i.e. flowingthrough with an inert gas, preferably nitrogen. It is also possible toreduce the concentration of dissolved oxygen by adding a reducing agent.The oxygen content of the monomer solution is preferably lowered beforethe polymerization to less than 1 ppm by weight, more preferably to lessthan 0.5 ppm by weight.

The water content of the monomer solution is preferably less than 65% byweight, preferentially less than 62% by weight, more preferably lessthan 60% by weight, most preferably less than 58% by weight.

The monomer solution has, at 20° C., a dynamic viscosity of preferablyfrom 0.002 to 0.02 Pa·s, more preferably from 0.004 to 0.015 Pas, mostpreferably from 0.005 to 0.01 Pa-s. The mean droplet diameter in thedroplet generation rises with rising dynamic viscosity.

The monomer solution has, at 20° C., a density of preferably from 1 to1.3 g/cm³, more preferably from 1.05 to 1.25 g/cm³, most preferably from1.1 to 1.2 g/cm³.

The monomer solution has, at 20° C., a surface tension of from 0.02 to0.06 N/m, more preferably from 0.03 to 0.05 N/m, most preferably from0.035 to 0.045 N/m. The mean droplet diameter in the droplet generationrises with rising surface tension.

Polymerization

The monomer solution is polymerized. Suitable reactors are, for example,kneading reactors or belt reactors. In the kneader, the polymer gelformed in the polymerization of an aqueous monomer solution orsuspension is comminuted continuously by, for example, contrarotatorystirrer shafts, as described in WO 2001/038402 A1. Polymerization on thebelt is described, for example, in DE 38 25 366 A1 and U.S. Pat. No.6,241,928. Polymerization in a belt reactor forms a polymer gel whichhas to be comminuted in a further process step, for example in anextruder or kneader.

To improve the drying properties, the comminuted polymer gel obtained bymeans of a kneader can additionally be extruded.

In a preferred embodiment of the present invention the water-absorbentpolymer particles are produced by polymerizing droplets of the monomerin a surrounding heated gas phase, for example using a system describedin WO 2008/040715 A2, WO 2008/052971 A1, WO 2008/069639 A1 and WO2008/086976 A1.

The droplets are preferably generated by means of a droplet plate. Adroplet plate is a plate having a multitude of bores, the liquidentering the bores from the top. The droplet plate or the liquid can beoscillated, which generates a chain of ideally monodisperse droplets ateach bore on the underside of the droplet plate. In a preferredembodiment, the droplet plate is not agitated.

Within the scope of the present invention it is also possible to use twoor more droplet plates with different bore diameters so that a range ofdesired particle sizes can be produced. It is preferable that eachdroplet plate carries only one bore diameter, however mixed borediameters in one plate are also possible.

The number and size of the bores are selected according to the desiredcapacity and droplet size. The droplet diameter is typically 1.9 timesthe diameter of the bore. What is important here is that the liquid tobe dropletized does not pass through the bore too rapidly and thepressure drop over the bore is not too great. Otherwise, the liquid isnot dropletized, but rather the liquid jet is broken up (sprayed) owingto the high kinetic energy. The Reynolds number based on the throughputper bore and the bore diameter is preferably less than 2000,preferentially less than 1600, more preferably less than 1400 and mostpreferably less than 1200.

The underside of the droplet plate has at least in part a contact anglepreferably of at least 60°, more preferably at least 75° and mostpreferably at least 90° with regard to water.

The contact angle is a measure of the wetting behavior of a liquid, inparticular water, with regard to a surface, and can be determined usingconventional methods, for example in accordance with ASTM D 5725. A lowcontact angle denotes good wetting, and a high contact angle denotespoor wetting.

It is also possible for the droplet plate to consist of a materialhaving a lower contact angle with regard to water, for example a steelhaving the German construction material code number of 1.4571, and becoated with a material having a larger contact angle with regard towater.

Useful coatings include for example fluorous polymers, such asperfluoroalkoxyethylene, polytetrafluoroethylene,ethylene-chlorotrifluoroethylene copolymers,ethylene-tetrafluoroethylene copolymers and fluorinated polyethylene.

The coatings can be applied to the substrate as a dispersion, in whichcase the solvent is subsequently evaporated off and the coating is heattreated. For polytetrafluoroethylene this is described for example inU.S. Pat. No. 3,243,321.

Further coating processes are to be found under the headword “ThinFilms” in the electronic version of “Ullmann's Encyclopedia ofIndustrial Chemistry” (Updated Sixth Edition, 2000 Electronic Release).

The coatings can further be incorporated in a nickel layer in the courseof a chemical nickelization.

It is the poor wettability of the droplet plate that leads to theproduction of monodisperse droplets of narrow droplet size distribution.

The droplet plate has preferably at least 5, more preferably at least25, most preferably at least 50 and preferably up to 750, morepreferably up to 500 bores, most preferably up to 250. The number ofbores is determined mainly by geometrical and manufacturing constraintsand can be adjusted to practical use conditions even outside the abovegiven range. The diameter of the bores is adjusted to the desireddroplet size.

The separation of the bores is usually from 5 to 50 mm, preferably from6 to 40 mm, more preferably from 7 to 35 mm, most preferably from 8 to30 mm. Smaller separations of the bores may cause agglomeration of thepolymerizing droplets.

The diameter of the bores is preferably from 50 to 500 μm, morepreferably from 100 to 300 μm, most preferably from 150 to 250 μm.

For optimizing the average particle diameter, droplet plates withdifferent bore diameters can be used. The variation can be done bydifferent bores on one plate or by using different plates, where eachplate has a different bore diameter. The average particle sizedistribution can be monomodal, bimodal or multimodal. Most preferably itis monomodal or bimodal.

The temperature of the monomer solution as it passes through the bore ispreferably from 5 to 80° C., more preferably from 10 to 70° C., mostpreferably from 30 to 60° C.

A gas flows through the reaction chamber. The carrier gas is conductedthrough the reaction chamber in cocurrent to the free-falling dropletsof the monomer solution, i.e. from the top downward. After one pass, thegas is preferably recycled at least partly, preferably to an extent ofat least 50%, more preferably to an extent of at least 75%, into thereaction chamber as cycle gas. Typically, a portion of the carrier gasis discharged after each pass, preferably up to 10%, more preferably upto 3% and most preferably up to 1%.

The carrier gas may be composed of air. The oxygen content of thecarrier gas is preferably from 0.1 to 15% by volume, more preferablyfrom 1 to 10% by volume, most preferably from 2 to 7% by weight. In thescope of the present invention it is also possible to use a carrier gaswhich is free of oxygen.

As well as oxygen, the carrier gas preferably comprises nitrogen. Thenitrogen content of the gas is preferably at least 80% by volume, morepreferably at least 90% by volume, most preferably at least 95% byvolume. Other possible carrier gases may be selected from carbondioxide, argon, xenon, krypton, neon, helium, sulfurhexafluoride. Anymixture of carrier gases may be used. The carrier gas may also becomeloaded with water and/or acrylic acid vapors.

The gas velocity is preferably adjusted such that the flow in thereaction zone is directed, for example no convection currents opposed tothe general flow direction are present, and is preferably from 0.1 to2.5 m/s, more preferably from 0.3 to 1.5 m/s, even more preferably from0.5 to 1.2 m/s, most preferably from 0.7 to 0.9 m/s.

The gas entrance temperature, i.e. the temperature with which the gasenters the reaction zone, is preferably from 160 to 200° C., morepreferably from 165 to 195° C., even more preferably from 170 to 190°C., most preferably from 175 to 185° C.

The steam content of the gas that enters the reaction zone is preferablyfrom 0.01 to 0.15 kg per kg dry gas, more preferably from 0.02 to 0.12kg per kg dry gas, most preferably from 0.03 to 0.10 kg per kg dry gas.

The gas entrance temperature is controlled in such a way that the gasexit temperature, i.e. the temperature with which the gas leaves thereaction zone, is less than 150° C., preferably from 90 to 140° C., morepreferably from 100 to 130° C., even more preferably from 105 to 125°C., most preferably from 110 to 120° C.

The steam content of the gas that leaves the reaction zone is preferablyfrom 0.02 to 0.30 kg per kg dry gas, more from 0.04 to 0.28 kg per kgdry gas, most from 0.05 to 0.25 kg per kg dry gas.

The water-absorbent polymer particles can be divided into threecategories: water-absorbent polymer particles of Type 1 are particleswith one cavity, water-absorbent polymer particles of Type 2 areparticles with more than one cavity, and water-absorbent polymerparticles of Type 3 are solid particles with no visible cavity. Type 1particles are represented by hollow-spheres, Type 2 particles arerepresented by spherical closed cell sponges, and Type 3 particles arerepresented by solid spheres. Type 2 or Type 3 particles or mixturesthereof with little or no Type 1 particles are preferred.

The morphology of the water-absorbent polymer particles can becontrolled by the reaction conditions during polymerization.Water-absorbent polymer particles having a high amount of particles withone cavity (Type 1) can be prepared by using low gas velocities and highgas exit temperatures. Water-absorbent polymer particles having a highamount of particles with more than one cavity (Type 2) can be preparedby using high gas velocities and low gas exit temperatures.

Water-absorbent polymer particles having no cavity (Type 3) andwater-absorbent polymer particles having more than one cavity (Type 2)show an improved mechanical stability compared with water-absorbentpolymer particles having only one cavity (Type 1).

As a particular advantage round shaped particles have no edges that caneasily be broken by processing stress in diaper production and duringswelling in aqueous liquid there are no breakpoints on the surface thatcould lead to loss of mechanical strength.

The reaction can be carried out under elevated pressure or under reducedpressure, preferably from 1 to 100 mbar below ambient pressure, morepreferably from 1.5 to 50 mbar below ambient pressure, most preferablyfrom 2 to 10 mbar below ambient pressure.

The reaction off-gas, i.e. the gas leaving the reaction chamber, may becooled in a heat exchanger. This condenses water and unconverted monomera). The reaction off-gas can then be reheated at least partly andrecycled into the reaction chamber as cycle gas. A portion of thereaction off-gas can be discharged and replaced by fresh gas, in whichcase water and unconverted monomers a) present in the reaction off-gascan be removed and recycled.

Particular preference is given to a thermally integrated system, i.e. aportion of the waste heat in the cooling of the off-gas is used to heatthe cycle gas.

The reactors can be trace-heated. In this case, the trace heating isadjusted such that the wall temperature is at least 5° C. above theinternal reactor temperature and condensation on the reactor walls isreliably prevented.

Thermal Posttreatment

The water-absorbent polymer particles obtained by dropletization may bethermal posttreated for adjusting the content of residual monomers tothe desired value.

The residual monomers can be removed better at relatively hightemperatures and relatively long residence times. What is important hereis that the water-absorbent polymer particles are not too dry. In thecase of excessively dry particles, the residual monomers decrease onlyinsignificantly. Too high a water content increases the caking tendencyof the water-absorbent polymer particles.

The thermal posttreatment can be done in a fluidized bed. In a preferredembodiment of the present invention an internal fluidized bed is used.An internal fluidized bed means that the product of the dropletizationpolymerization is accumulated in a fluidized bed below the reactionzone.

In the fluidized state, the kinetic energy of the polymer particles isgreater than the cohesion or adhesion potential between the polymerparticles.

The fluidized state can be achieved by a fluidized bed. In this bed,there is upward flow toward the water-absorbing polymer particles, sothat the particles form a fluidized bed. The height of the fluidized bedis adjusted by gas rate and gas velocity, i.e. via the pressure drop ofthe fluidized bed (kinetic energy of the gas).

The velocity of the gas stream in the fluidized bed is preferably from0.3 to 2.5 m/s, more preferably from 0.4 to 2.0 m/s, most preferablyfrom 0.5 to 1.5 m/s.

The pressure drop over the bottom of the internal fluidized bed ispreferably from 1 to 100 mbar, more preferably from 3 to 50 mbar, mostpreferably from 5 to 25 mbar.

The moisture content of the water-absorbent polymer particles at the endof the thermal posttreatment is preferably from 1 to 20% by weight, morepreferably from 2 to 15% by weight, even more preferably from 3 to 12%by weight, most preferably 5 to 8% by weight.

The temperature of the water-absorbent polymer particles during thethermal posttreatment is from 20 to 120° C., preferably from 40 to 100°C., more preferably from 50 to 95° C., even more preferably from 55 to90° C., most preferably from 60 to 80° C.

The average residence time in the internal fluidized bed is from 10 to300 minutes, preferably from 60 to 270 minutes, more preferably from 40to 250 minutes, most preferably from 120 to 240 minutes.

The condition of the fluidized bed can be adjusted for reducing theamount of residual monomers of the water-absorbent polymers leaving thefluidized bed. The amount of residual monomers can be reduced to levelsbelow 0.1% by weight by a thermal posttreatment using additional steam.

The steam content of the gas is preferably from 0.005 to 0.25 kg per kgof dry gas, more preferably from 0.01 to 0.2 kg per kg of dry gas, mostpreferably from 0.02 to 0.15 kg per kg of dry gas. By using additionalsteam the condition of the fluidized bed can be adjusted that the amountof residual monomers of the water-absorbent polymers leaving thefluidized bed is from 0.03 to 15% by weight, preferably from 0.05 to 12%by weight, more preferably from 0.1 to 10% by weight, even morepreferably from 0.15 to 7.5% by weight most preferably from 0.2 to 5% byweight, even most preferably from 0.25 to 2.5% by weight.

The level of residual monomers in the water-absorbent polymer has inimportant impact on the properties of the later formedsurface-postcrosslinked water-absorbent polymer particles. That meansthat very low levels of residual monomers must be avoided.

In one preferred embodiment of the present invention the thermalposttreatment is completely or at least partially done in an externalfluidized bed. The operating conditions of the external fluidized bedare within the scope for the internal fluidized bed as described above.

In another preferred embodiment of the present invention the thermalposttreatment is done in an external mixer with moving mixing tools,preferably horizontal mixers, such as screw mixers, disk mixers, screwbelt mixers and paddle mixers. Suitable mixers are, for example, Beckershovel mixers (Gebr. Lödige Maschinenbau GmbH; Paderborn; Germany), Narapaddle mixers (NARA Machinery Europe; Frechen; Germany), Pflugschar®plowshare mixers (Gebr. Lödige Maschinenbau GmbH; Paderborn; Germany),Vrieco-Nauta Continuous Mixers (Hosokawa Micron BV; Doetinchem; theNetherlands), Processall Mixmill Mixers (Processall Incorporated;Cincinnati; U.S.A.) and Ruberg continuous flow mixers (Gebrüder RubergGmbH & Co KG, Nieheim, Germany). Ruberg continuous flow mixers, Beckershovel mixers and Pflugschar® plowshare mixers are preferred.

The thermal posttreatment can be done in a discontinuous external mixeror a continuous external mixer.

The amount of gas to be used in the discontinuous external mixer ispreferably from 0.01 to 5 Nm³/h, more preferably from 0.05 to 2 Nm³/h,most preferably from 0.1 to 0.5 Nm³/h, based in each case on kgwater-absorbent polymer particles.

The amount of gas to be used in the continuous external mixer ispreferably from 0.01 to 5 Nm³/h, more preferably from 0.05 to 2 Nm³/h,most preferably from 0.1 to 0.5 Nm³/h, based in each case on kg/hthroughput of water-absorbent polymer particles.

The other constituents of the gas are preferably nitrogen, carbondioxide, argon, xenon, krypton, neon, helium, air or air/nitrogenmixtures, more preferably nitrogen or air/nitrogen mixtures comprisingless than 10% by volume of oxygen. Oxygen may cause discoloration.

The morphology of the water-absorbent polymer particles can also becontrolled by the reaction conditions during thermal posttreatment.Water-absorbent polymer particles having a high amount of particles withone cavity (Type 1) can be prepared by using high product temperaturesand short residence times. Water-absorbent polymer particles having ahigh amount of particles with more than one cavity (Type 2) can beprepared by using low product temperatures and long residence times.

Surface-Postcrosslinking

In the present invention the polymer particles aresurface-postcrosslinked for further improvement of the properties.

Surface-postcrosslinkers are compounds which comprise groups which canform at least two covalent bonds with the carboxylate groups of thepolymer particles. Suitable compounds are, for example, polyfunctionalamines, polyfunctional amidoamines, polyfunctional epoxides, asdescribed in EP 0 083 022 A2, EP 0 543 303 A1 and EP 0 937 736 A2, di-or polyfunctional alcohols as described in DE 33 14 019 A1, DE 35 23 617A1 and EP 0 450 922 A2, or β-hydroxyalkylamides, as described in DE 10204 938 A1 and U.S. Pat. No. 6,239,230. Also ethyleneoxide, aziridine,glycidol, oxetane and its derivatives may be used.

Polyvinylamine, polyamidoamines and polyvinylalcohole are examples ofmultifunctional polymeric surface-postcrosslinkers.

In addition, DE 40 20 780 C1 describes alkylene carbonates, DE 198 07502 A1 describes 1,3-oxazolidin-2-one and its derivatives such as2-hydroxyethyl-1,3-oxazolidin-2-one, DE 198 07 992 C1 describes bis- andpoly-1,3-oxazolidin-2-ones, EP 0 999 238 A1 describes bis- andpoly-1,3-oxazolidines, DE 198 54 573 A1 describes2-oxotetrahydro-1,3-oxazine and its derivatives, DE 198 54 574 A1describes N-acyl-1,3-oxazolidin-2-ones, DE 102 04 937 A1 describescyclic ureas, DE 103 34 584 A1 describes bicyclic amide acetals, EP 1199327 A2 describes oxetanes and cyclic ureas, and WO 2003/31482 A1describes morpholine-2,3-dione and its derivatives, as suitablesurface-postcrosslinkers.

In addition, it is also possible to use surface-postcrosslinkers whichcomprise additional polymerizable ethylenically unsaturated groups, asdescribed in DE 37 13 601 A1.

In a preferred embodiment of the present invention the at least onesurface-postcrosslinker is selected from alkylene carbonates,1,3-oxazolidin-2-ones, bis- and poly-1,3-oxazolidin-2-ones, bis- andpoly-1,3-oxazolidines, 2-oxotetrahydro-1,3-oxazines,N-acyl-1,3-oxazolidin-2-ones, cyclic ureas, bicyclic amide acetals,oxetanes, and morpholine-2,3-diones. Suitable surface-postcrosslinkersare ethylene carbonate, 3-methyl-1,3-oxazolidin-2-one,3-methyl-3-oxethanmethanol, 1,3-oxazolidin-2-one,3-(2-hydroxyethyl)-1,3-oxazolidin-2-one, 1,3-dioxan-2-one or a mixturethereof.

It is also possible to use any suitable mixture ofsurface-postcrosslinkers. It is particularly favorable to use mixturesof 1,3-dioxolan-2-on (ethylene carbonate) and 1,3-oxazolidin-2-ones.Such mixtures are obtainable by mixing and partly reacting of1,3-dioxolan-2-on (ethylene carbonate) with the corresponding2-amino-alcohol (e.g. 2-aminoethanol) and may comprise ethylene glycolfrom the reaction.

In a more preferred embodiment of the present invention at least onealkylene carbonate is used as surface-postcrosslinker. Suitable alkylenecarbonates are 1,3-dioxolan-2-on (ethylene carbonate),4-methyl-1,3-dioxolan-2-on (propylene carbonate),4,5-dimethyl-1,3-dioxolan-2-on, 4,4-dimethyl-1,3-dioxolan-2-on,4-ethyl-1,3-dioxolan-2-on, 4-hydroxymethyl-1,3-dioxolan-2-on (glycerinecarbonate), 1,3-dioxane-2-on (trimethylene carbonate),4-methyl-1,3-dioxane-2-on, 4,6-dimethyl-1,3-dioxane-2-on and1,3-dioxepan-2-on, preferably 1,3-dioxolan-2-on (ethylene carbonate) and1,3-dioxane-2-on (trimethylene carbonate), most preferably1,3-dioxolan-2-on (ethylene carbonate).

The amount of surface-postcrosslinker is preferably from 0.1 to 10% byweight, more preferably from 0.5 to 7.5% by weight, most preferably from1 to 5% by weight, based in each case on the polymer.

The content of residual monomers in the water-absorbent polymerparticles prior to the coating with the surface-postcrosslinker is inthe range from 0.03 to 15% by weight, preferably from 0.05 to 12% byweight, more preferably from 0.1 to 10% by weight, even more preferablyfrom 0.15 to 7.5% by weight, most preferably from 0.2 to 5% by weight,even most preferably from 0.25 to 2.5% by weight.

The moisture content of the water-absorbent polymer particles prior tothe thermal surface-postcrosslinking is preferably from 1 to 20% byweight, more preferably from 2 to 15% by weight, most preferably from 3to 10% by weight.

In a preferred embodiment of the present invention, polyvalent cationsare applied to the particle surface in addition to thesurface-postcrosslinkers before, during or after the thermalsurface-postcrosslinking.

The polyvalent cations usable in the process according to the inventionare, for example, divalent cations such as the cations of zinc,magnesium, calcium, iron and strontium, trivalent cations such as thecations of aluminum, iron, chromium, rare earths and manganese,tetravalent cations such as the cations of titanium and zirconium, andmixtures thereof. Possible counterions are chloride, bromide, sulfate,hydrogensulfate, methanesulfate, carbonate, hydrogencarbonate, nitrate,hydroxide, phosphate, hydrogenphosphate, dihydrogenphosphate,glycophosphate and carboxylate, such as acetate, glycolate, tartrate,formiate, propionate, 3-hydroxypropionate, lactamide and lactate, andmixtures thereof. Aluminum sulfate, aluminum acetate, and aluminumlactate are preferred. Aluminum lactate is more preferred. Using theinventive process in combination with the use of aluminum lactate,water-absorbent polymer particles having an extremely high total liquiduptake at lower centrifuge retention capacities (CRC) can be prepared.

Apart from metal salts, it is also possible to use polyamines and/orpolymeric amines as polyvalent cations. A single metal salt can be usedas well as any mixture of the metal salts and/or the polyamines above.

Preferred polyvalent cations and corresponding anions are disclosed inWO 2012/045705 A1 and are expressly incorporated herein by reference.Preferred polyvinylamines are disclosed in WO 2004/024816 A1 and areexpressly incorporated herein by reference.

The amount of polyvalent cation used is, for example, from 0.001 to 1.5%by weight, preferably from 0.005 to 1% by weight, more preferably from0.02 to 0.8% by weight, based in each case on the polymer.

The addition of the polyvalent metal cation can take place prior, after,or cocurrently with the surface-postcrosslinking. Depending on theformulation and operating conditions employed it is possible to obtain ahomogeneous surface coating and distribution of the polyvalent cation oran inhomogenous typically spotty coating. Both types of coatings and anymixes between them are useful within the scope of the present invention.

The surface-postcrosslinking is typically performed in such a way that asolution of the surface-postcrosslinker is sprayed onto the hydrogel orthe dry polymer particles. After the spraying, the polymer particlescoated with the surface-postcrosslinker are dried thermally and cooled.

The spraying of a solution of the surface-postcrosslinker is preferablyperformed in mixers with moving mixing tools, such as screw mixers, diskmixers and paddle mixers. Suitable mixers are, for example, verticalSchugi Flexomix® mixers (Hosokawa Micron BV; Doetinchem; theNetherlands), Turbolizers® mixers (Hosokawa Micron BV; Doetinchem; theNetherlands), horizontal Pflugschar® plowshare mixers (Gebr. LödigeMaschinenbau GmbH; Paderborn; Germany), Vrieco-Nauta Continuous Mixers(Hosokawa Micron BV; Doetinchem; the Netherlands), Processall MixmillMixers (Processall Incorporated; Cincinnati; US) and Ruberg continuousflow mixers (Gebrüder Ruberg GmbH & Co KG, Nieheim, Germany). Rubergcontinuous flow mixers and horizontal Pflugschar® plowshare mixers arepreferred. The surface-postcrosslinker solution can also be sprayed intoa fluidized bed.

The solution of the surface-postcrosslinker can also be sprayed on thewater-absorbent polymer particles during the thermal posttreatment. Insuch case the surface-postcrosslinker can be added as one portion or inseveral portions along the axis of thermal posttreatment mixer. In oneembodiment it is preferred to add the surface-postcrosslinker at the endof the thermal post-treatment step. As a particular advantage of addingthe solution of the surface-postcrosslinker during the thermalposttreatment step it may be possible to eliminate or reduce thetechnical effort for a separate surface-postcrosslinker addition mixer.

The surface-postcrosslinkers are typically used as an aqueous solution.The addition of nonaqueous solvent can be used to improve surfacewetting and to adjust the penetration depth of thesurface-postcrosslinker into the polymer particles.

The thermal surface-postcrosslinking is preferably carried out incontact dryers, more preferably paddle dryers, most preferably diskdryers. Suitable driers are, for example, Hosokawa Bepex® horizontalpaddle driers (Hosokawa Micron GmbH; Leingarten; Germany), HosokawaBepex® disk driers (Hosokawa Micron GmbH; Leingarten; Germany),Holo-Flite® dryers (Metso Minerals Industries Inc.; Danville; U.S.A.)and Nara paddle driers (NARA Machinery Europe; Frechen; Germany).Moreover, it is also possible to use fluidized bed dryers. In the lattercase the reaction times may be shorter compared to other embodiments.

When a horizontal dryer is used then it is often advantageous to set thedryer up with an inclined angle of a few degrees vs. the earth surfacein order to impart proper product flow through the dryer. The angle canbe fixed or may be adjustable and is typically between 0 to 10 degrees,preferably 1 to 6 degrees, most preferably 2 to 4 degrees.

In one embodiment of the present invention a contact dryer is used thathas two different heating zones in one apparatus. For example Narapaddle driers are available with just one heated zone or alternativelywith two heated zones. The advantage of using a two or more heated zonedryer is that different phases of the thermal post-treatment and/or ofthe post-surface-crosslinking can be combined.

In one preferred embodiment of the present invention a contact dryerwith a hot first heating zone is used which is followed by a temperatureholding zone in the same dryer. This set up allows a quick rise of theproduct temperature and evaporation of surplus liquid in the firstheating zone, whereas the rest of the dryer is just holding the producttemperature stable to complete the reaction.

In another preferred embodiment of the present invention a contact dryerwith a warm first heating zone is used which is then followed by a hotheating zone. In the first warm zone the thermal post-treatment isaffected or completed whereas the surface-postcrosslinking takes placein the subsequential hot zone.

In a typical embodiment a paddle heater with just one temperature zoneis employed.

A person skilled in the art will depending on the desired finishedproduct properties and the available base polymer qualities from thepolymerization step choose any one of these set ups. The thermalsurface-postcrosslinking can be effected in the mixer itself, by heatingthe jacket, blowing in warm air or steam. Equally suitable is adownstream dryer, for example a shelf dryer, a rotary tube oven or aheatable screw. It is particularly advantageous to mix and dry in afluidized bed dryer.

Preferred thermal surface-postcrosslinking temperatures are in the rangefrom 100 to 180° C., preferably from 120 to 170° C., more preferablyfrom 130 to 165° C., most preferably from 140 to 160° C. The preferredresidence time at this temperature in the reaction mixer or dryer ispreferably at least 5 minutes, more preferably at least 20 minutes, mostpreferably at least 40 minutes, and typically at most 120 minutes.

It is preferable to cool the polymer particles after thermalsurface-postcrosslinking. The cooling is preferably carried out incontact coolers, more preferably paddle coolers, most preferably diskcoolers. Suitable coolers are, for example, Hosokawa Bepex® horizontalpaddle coolers (Hosokawa Micron GmbH; Leingarten; Germany), HosokawaBepex® disk coolers (Hosokawa Micron GmbH; Leingarten; Germany),Holo-Flite® coolers (Metso Minerals Industries Inc.; Danville; U.S.A.)and Nara paddle coolers (NARA Machinery Europe; Frechen; Germany).Moreover, it is also possible to use fluidized bed coolers.

In the cooler the polymer particles are cooled to temperatures in therange from 20 to 150° C., preferably from 40 to 120° C., more preferablyfrom 60 to 100° C., most preferably from 70 to 90° C. Cooling using warmwater is preferred, especially when contact coolers are used.

Coating

To improve the properties, the water-absorbent polymer particles can becoated and/or optionally moistened. The internal fluidized bed, theexternal fluidized bed and/or the external mixer used for the thermalposttreatment and/or a separate coater (mixer) can be used for coatingof the water-absorbent polymer particles. Further, the cooler and/or aseparate coater (mixer) can be used for coating/moistening of thesurface-postcrosslinked water-absorbent polymer particles. Suitablecoatings for controlling the acquisition behavior and improving thepermeability (SFC or GBP) are, for example, inorganic inert substances,such as water-insoluble metal salts, organic polymers, cationicpolymers, anionic polymers and polyvalent metal cations. Suitablecoatings for improving the color stability are, for example reducingagents, chelating agents and anti-oxidants. Suitable coatings for dustbinding are, for example, polyols. Suitable coatings against theundesired caking tendency of the polymer particles are, for example,fumed silica, such as Aerosil® 200, and surfactants, such as Span® 20and Plantacare® 818 UP. Preferred coatings are aluminium dihydroxymonoacetate, aluminium sulfate, aluminium lactate, aluminium3-hydroxypropionate, zirconium acetate, citric acid or its water solublesalts, di- and monophosphoric acid or their water soluble salts,Blancolen®, Brüggolite® FF7, Cublen®, Span® 20 and Plantacare® 818 UP.

If salts of the above acids are used instead of the free acids then thepreferred salts are alkali-metal, earth alkali metal, aluminum,zirconium, titanium, zinc and ammonium salts.

Under the trade name Cublen® (Zschimmer & Schwarz Mohsdorf GmbH & Co KG;Burgstädt; Germany) the following acids and/or their alkali metal salts(preferably Na and K-salts) are available and may be used within thescope of the present invention for example to impart color-stability tothe finished product:

1-Hydroxyethane-1,1-diphosphonic acid, Amino-tris(methylene phosphonicacid), Ethylenediamine-tetra(methylene phosphonic acid),Diethylenetriamine-penta(methylene phosphonic acid), Hexamethylenediamine-tetra(methylenephosphonic acid), Hydroxyethyl-amino-di(methylenephosphonic acid), 2-Phosphonobutane-1,2,4-tricarboxylic acid,Bis(hexamethylenetriamine penta(methylene phosphonic acid).

Most preferably 1-Hydroxyethane-1,1-diphosphonic acid or its salts withsodium, potassium, or ammonium are employed. Any mixture of the aboveCublenes® can be used.

Alternatively, any of the chelating agents described before for use inthe polymerization can be coated onto the finished product.

Suitable inorganic inert substances are silicates such asmontmorillonite, kaolinite and talc, zeolites, activated carbons,polysilicic acids, magnesium carbonate, calcium carbonate, calciumphosphate, aluminum phosphate, barium sulfate, aluminum oxide, titaniumdioxide and iron(II) oxide. Preference is given to using polysilicicacids, which are divided between precipitated silicas and fumed silicasaccording to their mode of preparation. The two variants arecommercially available under the names Silica FK, Sipernat®, Wessalon®(precipitated silicas) and Aerosil® (fumed silicas) respectively. Theinorganic inert substances may be used as dispersion in an aqueous orwater-miscible dispersant or in substance.

When the water-absorbent polymer particles are coated with inorganicinert substances, the amount of inorganic inert substances used, basedon the water-absorbent polymer particles, is preferably from 0.05 to 5%by weight, more preferably from 0.1 to 1.5% by weight, most preferablyfrom 0.3 to 1% by weight.

Suitable organic polymers are polyalkyl methacrylates or thermoplasticssuch as polyvinyl chloride, waxes based on polyethylene, polypropylene,polyamides or polytetrafluoro-ethylene. Other examples arestyrene-isoprene-styrene block-copolymers or styrene-butadiene-styreneblock-copolymers. Another example are silanole-group bearingpolyvinylalcoholes available under the trade name Poval® R (KurarayEurope GmbH; Frankfurt; Germany).

Suitable cationic polymers are polyalkylenepolyamines, cationicderivatives of polyacrylamides, polyethyleneimines and polyquaternaryamines.

Polyquaternary amines are, for example, condensation products ofhexamethylenediamine, dimethylamine and epichlorohydrin, condensationproducts of dimethylamine and epichlorohydrin, copolymers ofhydroxyethylcellulose and diallyldimethylammonium chloride, copolymersof acrylamide and α-methacryloyloxyethyltrimethylammonium chloride,condensation products of hydroxyethylcellulose, epichlorohydrin andtrimethylamine, homopolymers of diallyldimethylammonium chloride andaddition products of epichlorohydrin to amidoamines. In addition,polyquaternary amines can be obtained by reacting dimethyl sulfate withpolymers such as polyethyleneimines, copolymers of vinylpyrrolidone anddimethylaminoethyl methacrylate or copolymers of ethyl methacrylate anddiethylaminoethyl methacrylate. The polyquaternary amines are availablewithin a wide molecular weight range.

However, it is also possible to generate the cationic polymers on theparticle surface, either through reagents which can form a network withthemselves, such as addition products of epichlorohydrin topolyamidoamines, or through the application of cationic polymers whichcan react with an added crosslinker, such as polyamines or polyimines incombination with polyepoxides, polyfunctional esters, polyfunctionalacids or polyfunctional (meth)acrylates.

It is possible to use all polyfunctional amines having primary orsecondary amino groups, such as polyethyleneimine, polyallylamine andpolylysine. The liquid sprayed by the process according to the inventionpreferably comprises at least one polyamine, for example polyvinylamineor a partially hydrolyzed polyvinylformamide.

The cationic polymers may be used as a solution in an aqueous orwater-miscible solvent as dispersion in an aqueous or water-miscibledispersant or in substance.

When the water-absorbent polymer particles are coated with a cationicpolymer, the use amount of cationic polymer based on the water-absorbentpolymer particles is usually not less than 0.001% by weight, typicallynot less than 0.01% by weight, preferably from 0.1 to 15% by weight,more preferably from 0.5 to 10% by weight, most preferably from 1 to 5%by weight.

Suitable anionic polymers are polyacrylates (in acidic form or partiallyneutralized as salt), copolymers of acrylic acid and maleic acidavailable under the trade name Sokalan® (BASF SE; Ludwigshafen;Germany), and polyvinylalcohols with built in ionic charges availableunder the trade name Poval® K (Kuraray Europe GmbH; Frankfurt; Germany).

Suitable polyvalent metal cations are Mg²⁺, Ca²⁺, Al³⁺, Sc³⁺, Ti⁴⁺,Mn²⁺, Fe^(2+/3+), Co²⁺, Ni²⁺, Cu^(+/2+), Zn²⁺, Y³⁺, Zr⁴⁺, Ag⁺, La³⁺,Ce⁴⁺, Hf⁴⁺ and Au^(+/3+); preferred metal cations are Mg²⁺, Ca²⁺, Al³⁺,Ti⁴⁺, Zr⁴⁺ and La³⁺; particularly preferred metal cations are Al³⁺, Ti⁴⁺and Zr⁴⁺. The metal cations may be used either alone or in a mixturewith one another. Suitable metal salts of the metal cations mentionedare all of those which have a sufficient solubility in the solvent to beused. Particularly suitable metal salts have weakly complexing anions,such as chloride, hydroxide, carbonate, acetate, formiate, propionate,nitrate, sulfate and methanesulfate. The metal salts are preferably usedas a solution or as a stable aqueous colloidal dispersion. The solventsused for the metal salts may be water, alcohols, ethylenecarbonate,propylenecarbonate, dimethylformamide, dimethyl sulfoxide and mixturesthereof. Particular preference is given to water and water/alcoholmixtures, such as water/methanol, water/isopropanol,water/1,3-propanediole, water/1,2-propandiole/1,4-butanediole orwater/propylene glycol.

When the water-absorbent polymer particles are coated with a polyvalentmetal cation, the amount of polyvalent metal cation used, based on thewater-absorbent polymer particles, is preferably from 0.05 to 5% byweight, more preferably from 0.1 to 1.5% by weight, most preferably from0.3 to 1% by weight.

Suitable reducing agents are, for example, sodium sulfite, sodiumhydrogensulfite (sodium bisulfite), sodium dithionite, sulfinic acidsand salts thereof, ascorbic acid, sodium hypophosphite, sodiumphosphite, and phosphinic acids and salts thereof. Preference is given,however, to salts of hypophosphorous acid, for example sodiumhypophosphite, salts of sulfinic acids, for example the disodium salt of2-hydroxy-2-sulfinatoacetic acid, and addition products of aldehydes,for example the disodium salt of 2-hydroxy-2-sulfonatoacetic acid. Thereducing agent used can be, however, a mixture of the sodium salt of2-hydroxy-2-sulfinatoacetic acid, the disodium salt of2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite. Such mixtures areobtainable as Brüggolite® FF6 and Bruggolite® FF7 (Brüggemann Chemicals;Heilbronn; Germany). Also useful is the purified2-hydroxy-2-sulfonatoacetic acid and its sodium salts, available underthe trade name Blancolen® from the same company.

The reducing agents are typically used in the form of a solution in asuitable solvent, preferably water. The reducing agent may be used as apure substance or any mixture of the above reducing agents may be used.

When the water-absorbent polymer particles are coated with a reducingagent, the amount of reducing agent used, based on the water-absorbentpolymer particles, is preferably from 0.01 to 5% by weight, morepreferably from 0.05 to 2% by weight, most preferably from 0.1 to 1% byweight.

Suitable polyols are polyethylene glycols having a molecular weight offrom 400 to 20000 g/mol, polyglycerol, 3- to 100-tuply ethoxylatedpolyols, such as trimethylolpropane, glycerol, sorbitol, mannitol,inositol, pentaerythritol and neopentyl glycol. Particularly suitablepolyols are 7- to 20-tuply ethoxylated glycerol or trimethylolpropane,for example Polyol TP 70® (Perstorp AB, Perstorp, Sweden). The latterhave the advantage in particular that they lower the surface tension ofan aqueous extract of the water-absorbent polymer particles onlyinsignificantly. The polyols are preferably used as a solution inaqueous or water-miscible solvents.

The polyol can be added before, during, or after surface-crosslinking.Preferably it is added after surface-cross linking. Any mixture of theabove listed polyols may be used.

When the water-absorbent polymer particles are coated with a polyol, theuse amount of polyol, based on the water-absorbent polymer particles, ispreferably from 0.005 to 2% by weight, more preferably from 0.01 to 1%by weight, most preferably from 0.05 to 0.5% by weight.

The coating is preferably performed in mixers with moving mixing tools,such as screw mixers, disk mixers, paddle mixers and drum coater.Suitable mixers are, for example, horizontal Pflugschar® plowsharemixers (Gebr. Lödige Maschinenbau GmbH; Paderborn; Germany),Vrieco-Nauta Continuous Mixers (Hosokawa Micron BV; Doetinchem; theNetherlands), Processall Mixmill Mixers (Processall Incorporated;Cincinnati; US) and Ruberg continuous flow mixers (Gebrüder Ruberg GmbH& Co KG, Nieheim, Germany). Moreover, it is also possible to use afluidized bed for mixing.

Agglomeration

The water-absorbent polymer particles can further selectively beagglomerated. The agglomeration can take place after the polymerization,the thermal postreatment, the thermal surface-postcrosslinking or thecoating.

Useful agglomeration assistants include water and water-miscible organicsolvents, such as alcohols, tetrahydrofuran and acetone; water-solublepolymers can be used in addition.

For agglomeration a solution comprising the agglomeration assistant issprayed onto the water-absorbing polymeric particles. The spraying withthe solution can, for example, be carried out in mixers having movingmixing implements, such as screw mixers, paddle mixers, disk mixers,plowshare mixers and shovel mixers. Useful mixers include for exampleLödige® mixers, Bepex® mixers, Nauta® mixers, Processall® mixers andSchugi® mixers. Vertical mixers are preferred. Fluidized bed apparatusesare particularly preferred.

Combination of Thermal Posttreatment, Surface-Postcrosslinking andOptionally Coating

In a preferred embodiment of the present invention the steps of thermalposttreatment and thermal surface-postcrosslinking are combined in oneprocess step. Such combination allows the use of low cost equipment andmoreover the process can be run at low temperatures, that iscost-efficient and avoids discoloration and loss of performanceproperties of the finished product by thermal degradation.

The mixer may be selected from any of the equipment options cited in thethermal posttreatment section. Ruberg continuous flow mixers, Beckershovel mixers and Pflugschar® plowshare mixers are preferred.

In this particular preferred embodiment the surface-postcrosslinkingsolution is sprayed onto the water-absorbent polymer particles underagitation.

Following the thermal posttreatment/surface-postcrosslinking thewater-absorbent polymer particles are dried to the desired moisturelevel and for this step any dryer cited in the surface-postcrosslinkingsection may be selected. However, as only drying needs to beaccomplished in this particular preferred embodiment it is possible touse simple and low cost heated contact dryers like a heated screw dryer,for example a Holo-Flite® dryer (Metso Minerals Industries Inc.;Danville; U.S.A.). Alternatively a fluidized bed may be used. In caseswhere the product needs to be dried with a predetermined and narrowresidence time it is possible to use torus disc dryers or paddle dryers,for example a Nara paddle dryer (NARA Machinery Europe; Frechen;Germany).

In a preferred embodiment of the present invention, polyvalent cationscited in the surface-postcrosslinking section are applied to theparticle surface before, during or after addition of thesurface-postcrosslinker by using different addition points along theaxis of a horizontal mixer.

In a very particular preferred embodiment of the present invention thesteps of thermal posttreatment, surface-postcrosslinking, and coatingare combined in one process step. Suitable coatings are cationicpolymers, surfactants, and inorganic inert substances that are cited inthe coating section. The coating agent can be applied to the particlesurface before, during or after addition of the surface-postcrosslinkeralso by using different addition points along the axis of a horizontalmixer.

The polyvalent cations and/or the cationic polymers can act asadditional scavengers for residual surface-postcrosslinkers. In apreferred embodiment of the present invention thesurface-postcrosslinkers are added prior to the polyvalent cationsand/or the cationic polymers to allow the surface-postcrosslinker toreact first.

The surfactants and/or the inorganic inert substances can be used toavoid sticking or caking during this process step under humidatmospheric conditions. Preferred surfactants are non-ionic andamphoteric surfactants. Preferred inorganic inert substances areprecipitated silicas and fumed silicas in form of powder or dispersion.

The amount of total liquid used for preparing the solutions/dispersionsis typically from 0.01% to 25% by weight, preferably from 0.5% to 12% byweight, more preferably from 2% to 7% by weight, most preferably from 3%to 6% by weight, in respect to the weight amount of water-absorbentpolymer particles to be processed.

Preferred embodiments are depicted in FIGS. 1 to 12.

FIG. 1: Process scheme (without external fluidized bed)

FIG. 2: Process scheme (with external fluidized bed)

FIG. 3: Arrangement of the T_outlet measurement

FIG. 4: Arrangement of the dropletizer units

FIG. 5: Dropletizer unit (longitudinal cut)

FIG. 6: Dropletizer unit (cross sectional view)

FIG. 7: Bottom of the internal fluidized bed (top view)

FIG. 8: openings in the bottom of the internal fluidized bed

FIG. 9: Rake stirrer for the intern fluidized bed (top view)

FIG. 10: Rake stirrer for the intern fluidized bed (cross sectionalview)

FIG. 11: Process scheme (surface-postcrosslinking)

FIG. 12: Process scheme (surface-postcrosslinking and coating)

FIG. 13: Contact dryer for surface-postcrosslnking

The reference numerals have the following meanings:

-   -   1 Drying gas inlet pipe    -   2 Drying gas amount measurement    -   3 Gas distributor    -   4 Dropletizer units    -   5 Cocurrent spray dryer, cylindrical part    -   6 Cone    -   7 T_outlet measurement    -   8 Tower offgas pipe    -   9 Baghouse filter    -   10 Ventilator    -   11 Quench nozzles    -   12 Condenser column, counter current cooling    -   13 Heat exchanger    -   14 Pump    -   15 Pump    -   16 Water outlet    -   17 Ventilator    -   18 Offgas outlet    -   19 Nitrogen inlet    -   20 Heat exchanger    -   21 Ventilator    -   22 Heat exchanger    -   23 Steam injection via nozzles    -   24 Water loading measurement    -   25 Conditioned internal fluidized bed gas    -   26 Internal fluidized bed product temperature measurement    -   27 Internal fluidized bed    -   28 Rotary valve    -   29 Sieve    -   30 End product    -   31 Static mixer    -   32 Static mixer    -   33 Initiator feed    -   34 Initiator feed    -   35 Monomer feed    -   36 Fine particle fraction outlet to rework    -   37 External fluidized bed    -   38 Ventilator    -   39 External fluidized bed offgas outlet to baghouse filter    -   40 Rotary valve    -   41 Filtered air inlet    -   42 Ventilator    -   43 Heat exchanger    -   44 Steam injection via nozzle    -   45 Water loading measurement    -   46 Conditioned external fluidized bed gas    -   47 T_-outlet measurement (average temperature out of 3        measurements around tower circumference)    -   48 Dropletizer unit    -   49 Monomer premixed with initiator feed    -   50 Spray dryer tower wall    -   51 Dropletizer unit outer pipe    -   52 Dropletizer unit inner pipe    -   53 Dropletizer cassette    -   54 Teflon block    -   55 Valve    -   56 Monomer premixed with initiator feed inlet pipe connector    -   57 Droplet plate    -   58 Counter plate    -   59 Flow channels for temperature control water    -   60 Dead volume free flow channel for monomer solution    -   61 Dropletizer cassette stainless steel block    -   62 Bottom of the internal fluidized bed with four segments    -   63 Split openings of the segments    -   64 Rake stirrer    -   65 Prongs of the rake stirrer    -   66 Mixer    -   67 Optional coating feed    -   68 Postcrosslinker feed    -   69 Thermal dryer (surface-postcrosslinking)    -   70 Cooler    -   71 Optional coating/water feed    -   72 Coater    -   73 Coating/water feed    -   74 Base polymer feed    -   75 Discharge zone    -   76 Weir opening    -   77 weir plate    -   78 Weir height 100%    -   79 Weir height 50%    -   80 Shaft    -   81 Discharge cone    -   82 Inclination angle α    -   83 Temperature sensors (T₁ to T₆)    -   84 Paddle (shaft offset 90°)

The drying gas is fed via a gas distributor (3) at the top of the spraydryer as shown in FIG. 1. The drying gas is partly recycled (drying gasloop) via a baghouse filter (9) and a condenser column (12). Thepressure inside the spray dryer is below ambient pressure.

The spray dryer outlet temperature is preferably measured at threepoints around the circumference at the end of the cylindrical part asshown in FIG. 3. The single measurements (47) are used to calculate theaverage cylindrical spray dryer outlet temperature.

The product accumulated in the internal fluidized bed (27). Conditionedinternal fluidized bed gas is fed to the internal fluidized bed (27) vialine (25). The relative humidity of the internal fluidized bed gas ispreferably controlled by adding steam via line (23).

The spray dryer offgas is filtered in baghouse filter (9) and sent to acondenser column (12) for quenching/cooling. After the baghouse filter(9) a recuperation heat exchanger system for preheating the gas afterthe condenser column (12) can be used. The baghouse filter (9) may betrace-heated on a temperature of preferably from 80 to 180° C., morepreferably from 90 to 150° C., most preferably from 100 to 140° C.Excess water is pumped out of the condenser column (12) by controllingthe (constant) filling level inside the condenser column (12). The waterinside the condenser column (12) is cooled by a heat exchanger (13) andpumped counter-current to the gas via quench nozzles (11) so that thetemperature inside the condenser column (12) is preferably from 20 to100° C., more preferably from 25 to 80° C., most preferably from 30 to60° C. The water inside the condenser column (12) is set to an alkalinepH by dosing a neutralizing agent to wash out vapors of monomer a).Aqueous solution from the condenser column (12) can be sent back forpreparation of the monomer solution.

The condenser column offgas is split to the drying gas inlet pipe (1)and the conditioned internal fluidized bed gas (25). The gastemperatures are controlled via heat exchangers (20) and (22). The hotdrying gas is fed to the cocurrent spray dryer via gas distributor (3).The gas distributor (3) consists preferably of a set of plates providinga pressure drop of preferably 1 to 100 mbar, more preferably 2 to 30mbar, most preferably 4 to 20 mbar, depending on the drying gas amount.Turbulences and/or a centrifugal velocity can also be introduced intothe drying gas if desired by using gas nozzles or baffle plates.

Conditioned internal fluidized bed gas is fed to the internal fluidizedbed (27) via line (25). The relative humidity of the external fluidizedbed gas is preferably controlled by adding steam via line (23). Toprevent any condensation the steam is added together with the internalfluidized bed into the heat exchanger (22). The product holdup in theinternal fluidized bed (27) can be controlled via rotational speed ofthe rotary valve (28).

The product is discharged from the internal fluidized bed (27) viarotary valve (28). The product holdup in the internal fluidized bed (27)can be controlled via rotational speed of the rotary valve (28). Thesieve (29) is used for sieving off overs/lumps.

The monomer solution is preferably prepared by mixing first monomer a)with a neutralization agent and optionally secondly with crosslinker b).The temperature during neutralization is controlled to preferably from 5to 60° C., more preferably from 8 to 40° C., most preferably from 10 to30° C., by using a heat exchanger and pumping in a loop. A filter unitis preferably used in the loop after the pump. The initiators aremetered into the monomer solution upstream of the dropletizer by meansof static mixers (31) and (32) via lines (33) and (34) as shown inFIG. 1. Preferably a peroxide solution having a temperature ofpreferably from 5 to 60° C., more preferably from 10 to 50° C., mostpreferably from 15 to 40° C., is added via line (33) and preferably anazo initiator solution having a temperature of preferably from 2 to 30°C., more preferably from 3 to 15° C., most preferably from 4 to 8° C.,is added via line (34). Each initiator is preferably pumped in a loopand dosed via control valves to each dropletizer unit. A second filterunit is preferably used after the static mixer (32). The mean residencetime of the monomer solution admixed with the full initiator package inthe piping before the droplet plates (57) is preferably less than 60 s,more preferably less than 30 s, most preferably less than 10 s.

For dosing the monomer solution into the top of the spray dryerpreferably three dropletizer units are used as shown in FIG. 4. However,any number of dropletizers can be used that is required to optimize thethroughput of the process and the quality of the product. Hence, in thepresent invention at least one dropletizer is employed, and as manydropletizers as geometrically allowed may be used.

A dropletizer unit consists of an outer pipe (51) having an opening forthe dropletizer cassette (53) as shown in FIG. 5. The dropletizercassette (53) is connected with an inner pipe (52). The inner pipe (53)having a PTFE block (54) at the end as sealing can be pushed in and outof the outer pipe (51) during operation of the process for maintenancepurposes.

The temperature of the dropletizer cassette (61) is controlled topreferably 5 to 80° C., more preferably 10 to 70° C., most preferably 30to 60° C., by water in flow channels (59) as shown in FIG. 6.

The dropletizer cassette has preferably from 10 to 1500, more preferablyfrom 50 to 1000, most preferably from 100 to 500, bores having adiameter of preferably from 50 to 500 μm, more preferably from 100 to300 μm, most preferably from 150 to 250 μm. The bores can be ofcircular, rectangular, triangular or any other shape. Circular bores arepreferred. The ratio of bore length to bore diameter is preferably from0.5 to 10, more preferably from 0.8 to 5, most preferably from 1 to 3.The droplet plate (57) can have a greater thickness than the bore lengthwhen using an inlet bore channel. The droplet plate (57) is preferablylong and narrow as disclosed in WO 2008/086976 A1. Multiple rows ofbores per droplet plate can be used, preferably from 1 to 20 rows, morepreferably from 2 to 5 rows.

The dropletizer cassette (61) consists of a flow channel (60) havingessential no stagnant volume for homogeneous distribution of thepremixed monomer and initiator solutions and two droplet plates (57).The droplet plates (57) have an angled configuration with an angle ofpreferably from 1 to 90°, more preferably from 3 to 45°, most preferablyfrom 5 to 20°. Each droplet plate (57) is preferably made of a heatand/or chemically resistant material, such as stainless steel, polyetherether ketone, polycarbonate, polyarylsulfone, such as polysulfone, orpolyphenylsulfone, or fluorous polymers, such asperfluoroalkoxyethylene, polytetrafluoroethylene, polyvinylidenfluorid,ethylene-chlorotrifluoroethylene copolymers,ethylene-tetrafluoroethylene copolymers and fluorinated polyethylene.Coated droplet plates as disclosed in WO 2007/031441 A1 can also beused. The choice of material for the droplet plate is not limited exceptthat droplet formation must work and it is preferable to use materialswhich do not catalyze the start of polymerization on its surface.

The throughput of monomer including initiator solutions per dropletizerunit is preferably from 150 to 2500 kg/h, more preferably from 200 to1000 kg/h, most preferably from 300 to 600 kg/h. The throughput per boreis preferably from 0.1 to 10 kg/h, more preferably from 0.5 to 5 kg/h,most preferably from 0.7 to 2 kg/h.

The start-up of the cocurrent spray dryer (5) can be done in thefollowing sequence:

-   -   starting the condenser column (12),    -   starting the ventilators (10) and (17),    -   starting the heat exchanger (20),    -   heating up the drying gas loop up to 95° C.,    -   starting the nitrogen feed via the nitrogen inlet (19),    -   waiting until the residual oxygen is below 4% by weight,    -   heating up the drying gas loop,    -   at a temperature of 105° C. starting the water feed (not shown)        and    -   at target temperature stopping the water feed and starting the        monomer feed via dropletizer unit (4)

The shut-down of the cocurrent spray dryer (5) can be done in thefollowing sequence:

-   -   stopping the monomer feed and starting the water feed (not        shown),    -   shut-down of the heat exchanger (20),    -   cooling the drying gas loop via heat exchanger (13),    -   at a temperature of 105° C. stopping the water feed,    -   at a temperature of 60° C. stopping the nitrogen feed via the        nitrogen inlet (19) and    -   feeding air into the drying gas loop (not shown)

To prevent damages the cocurrent spray dryer (5) must be heated up andcooled down very carefully. Any quick temperature change must beavoided.

The openings in the bottom of the internal fluidized bed may be arrangedin a way that the water-absorbent polymer particles flow in a cycle asshown in FIG. 7. The bottom shown in FIG. 7 comprises of four segments(62). The openings (63) in the segments (62) are in the shape of slitsthat guides the passing gas stream into the direction of the nextsegment (62). FIG. 8 shows an enlarged view of the openings (63).

The opening may have the shape of holes or slits. The diameter of theholes is preferred from 0.1 to 10 mm, more preferred from 0.2 to 5 mm,most preferred from 0.5 to 2 mm. The slits have a length of preferredfrom 1 to 100 mm, more preferred from 2 to 20 mm, most preferred from 5to 10 mm, and a width of preferred from 0.5 to 20 mm, more preferredfrom 1 to 10 mm, most preferred from 2 to 5 mm.

FIG. 9 and FIG. 10 show a rake stirrer (64) that may be used in theinternal fluidized bed. The prongs (65) of the rake have a staggeredarrangement. The speed of rake stirrer is preferably from 0.5 to 20 rpm,more preferably from 1 to 10 rpm most preferably from 2 to 5 rpm.

For start-up the internal fluidized bed may be filled with a layer ofwater-absorbent polymer particles, preferably 5 to 50 cm, morepreferably from 10 to 40 cm, most preferably from 15 to 30 cm.

Water-Absorbent Polymer Particles

The present invention provides water-absorbent polymer particlesobtainable by the process according to the invention.

The present invention further provides surface-postcrosslinkedwater-absorbent polymer particles having a centrifuge retention capacityfrom 35 to 75 g/g, an absorption under high load from 20 to 50 g/g, alevel of extractable constituents of less than 10% by weight, and aporosity from 20 to 40%.

It is particular advantageous that the surface-postcrosslinkedwater-absorbent polymer particles obtainable by the process according tothe invention exhibit a very high centrifuge retention capacity (CRC)and a high absorption under high load (AUHL), and that the sum of theseparameters (=CRC+AUHL) is at least 60 g/g, preferably at least 65 g/g,most preferably at least 70 g/g, and not more than 120 g/g, preferablyless than 100 g/g, more preferably less than 90 g/g, and most preferablyless than 80 g/g. The surface-postcrosslinked water-absorbent polymerparticles obtainable by the process according to the invention furtherpreferably exhibit an absorption under high load (AUHL) of at least 15g/g, preferably at least 18 g/g, more preferably at least 21 g/g, mostpreferably at least 25 g/g, and not more than 50 g/g.

As the centrifuge retention capacity (CRC) is the maximum waterretention capacity of the surface-postcrosslinked water-absorbentpolymer particles it is of interest to maximize this parameter. Howeverthe absorption under high load (AUHL) is important to allow thefiber-matrix in a hygiene article to open up pores during swelling toallow further liquid to pass easily through the article structure toenable rapid uptake of this liquid. Hence there is a need to maximizeboth parameters.

The inventive water-absorbent polymer particles have a centrifugeretention capacity (CRC) from 35 to 75 g/g, preferably from 37 to 65g/g, more preferably from 39 to 60 g/g, most preferably from 40 to 55g/g.

The inventive water-absorbent polymer particles have an absorbency undera load of 49.2 g/cm² (AUHL) from 20 to 50 g/g, preferably from 22 to 45g/g, more preferably from 24 to 40 g/g, most preferably from 25 to 35g/g.

The inventive water-absorbent polymer particles have a level ofextractable constituents of less than 10% by weight, preferably lessthan 8% by weight, more preferably less than 6% by weight, mostpreferably less than 5% by weight.

The inventive water-absorbent polymer particles have a porosity from 20to 40%, preferably from 22 to 38%, more preferably from 24 to 36%, mostpreferably from 25 to 35%.

Preferred water-absorbent polymer particles are polymer particles havinga centrifuge retention capacity (CRC) from 37 to 65 g/g, an absorptionunder high load (AUHL) from 22 to 45 g/g, a level of extractableconstituents of less than 8% by weight and a porosity from 22 to 45%.

More preferred water-absorbent polymer particles are polymer particleshaving a centrifuge retention capacity (CRC) from 39 to 60 g/g, anabsorption under high load (AUHL) from 24 to 40 g/g, a level ofextractable constituents of less than 6% by weight and a porosity from24 to 40%.

Most preferred water-absorbent polymer particles are polymer particleshaving a centrifuge retention capacity (CRC) from 40 to 55 g/g, anabsorption under high load (AUHL) from 25 to 35 g/g, a level ofextractable constituents of less than 5% by weight and a porosity from25 to 35%.

The present invention further provides surface-postcrosslinkedwater-absorbent polymer particles having a total liquid uptake ofY>−500×ln(X)+1880,preferably Y>−495×ln(X)+1875,more preferably Y>−490×ln(X)+1870,most preferably Y>−485×ln(X)+1865,wherein Y [g] is the total liquid uptake and X [g/g] is the centrifugeretention capacity (CRC), wherein the centrifuge retention capacity(CRC) is at least 25 g/g, preferably at least 30 g/g, more preferably atleast 35 g/g, most preferably at least 40 g/g, and the liquid uptake isat least 30 g, preferably at least 35 g/g, more preferably at least 40g/g, most preferably at least 45 g/g.

The present invention further provides surface-postcrosslinkedwater-absorbent polymer particles having a change of characteristicswelling time of less than 0.6, preferably less than 0.5, morepreferably less than 0.45, most preferably less than 0.4, and acentrifuge retention capacity (CRC) of at least 35 g/g, preferably atleast 37 g/g, more preferably at least 38.5 g/g, most preferably atleast 40 g/g, wherein the change of characteristic swelling time isZ<(τ_(0.5)−τ_(0.1))/τ_(0.5)wherein Z is the change of characteristic swelling time, τ_(0.1) is thecharacteristic swelling time under a pressure of 0.1 psi (6.9 g/cm²) andτ_(0.5) is the characteristic swelling time under a pressure of 0.5 psi(35.0 g/cm²).

The inventive water-absorbent polymer particles have a mean sphericityfrom 0.80 to 0.95, preferably from 0.82 to 0.93, more preferably from0.84 to 0.91, most preferably from 0.85 to 0.90. The sphericity (SPHT)is defined as

${{SPHT} = \frac{4\pi\; A}{U^{2}}},$where A is the cross-sectional area and U is the cross-sectionalcircumference of the polymer particles. The mean sphericity is thevolume-average sphericity.

The mean sphericity can be determined, for example, with the Camsizer®image analysis system (Retsch Technology GmbH; Haan; Germany):

For the measurement, the product is introduced through a funnel andconveyed to the falling shaft with a metering channel. While theparticles fall past a light wall, they are recorded selectively by acamera. The images recorded are evaluated by the software in accordancewith the parameters selected.

To characterize the roundness, the parameters designated as sphericityin the program are employed. The parameters reported are the meanvolume-weighted sphericities, the volume of the particles beingdetermined via the equivalent diameter xc_(min). To determine theequivalent diameter xc_(min), the longest chord diameter for a total of32 different spatial directions is measured in each case. The equivalentdiameter xc_(min) is the shortest of these 32 chord diameters. To recordthe particles, the so-called CCD-zoom camera (CAM-Z) is used. To controlthe metering channel, a surface coverage fraction in the detectionwindow of the camera (transmission) of 0.5% is predefined.

Water-absorbent polymer particles with relatively low sphericity areobtained by reverse suspension polymerization when the polymer beads areagglomerated during or after the polymerization.

The water-absorbent polymer particles prepared by customary solutionpolymerization (gel polymerization) are ground and classified afterdrying to obtain irregular polymer particles. The mean sphericity ofthese polymer particles is between approx. 0.72 and approx. 0.78.

The inventive water-absorbent polymer particles have a content ofhydrophobic solvent of preferably less than 0.005% by weight, morepreferably less than 0.002% by weight and most preferably less than0.001% by weight. The content of hydrophobic solvent can be determinedby gas chromatography, for example by means of the headspace technique.A hydrophobic solvent within the scope of the present invention iseither immiscible in water or only sparingly miscible. Typical examplesof hydrophobic solvents are pentane, hexane, cyclohexane, toluene.

Water-absorbent polymer particles which have been obtained by reversesuspension polymerization still comprise typically approx. 0.01% byweight of the hydrophobic solvent used as the reaction medium.

The inventive water-absorbent polymer particles have a dispersantcontent of typically less than 1% by weight, preferably less than 0.5%by weight, more preferably less than 0.1% by weight and most preferablyless than 0.05% by weight.

Water-absorbent polymer particles which have been obtained by reversesuspension polymerization still comprise typically at least 1% by weightof the dispersant, i.e. ethylcellulose, used to stabilize thesuspension.

The inventive water-absorbent polymer particles have a bulk densitypreferably from 0.6 to 1 g/cm³, more preferably from 0.65 to 0.9 g/cm³,most preferably from 0.68 to 0.8 g/cm³.

The average particle diameter (APD) of the inventive water-absorbentparticles is preferably from 200 to 550 μm, more preferably from 250 to500 μm, most preferably from 350 to 450 μm.

The particle diameter distribution (PDD) of the inventivewater-absorbent particles is preferably less than 0.7, more preferablyless than 0.65, more preferably less than 0.6.

The inventive water-absorbent polymer particles can be mixed with otherwater-absorbent polymer particles prepared by other processes, i.e.solution polymerization.

Fluid-Absorbent Articles

The present invention further provides fluid-absorbent articles. Thefluid-absorbent articles comprise of

-   -   (A) an upper liquid-pervious layer    -   (B) a lower liquid-impervious layer    -   (C) a fluid-absorbent core between (A) and (B) comprising        -   from 5 to 90% by weight fibrous material and from 10 to 95%            by weight water-absorbent polymer particles of the present            invention;        -   preferably from 20 to 80% by weight fibrous material and            from 20 to 80% by weight water-absorbent polymer particles            of the present invention;        -   more preferably from 30 to 75% by weight fibrous material            and from 25 to 70% by weight water-absorbent polymer            particles of the present invention;        -   most preferably from 40 to 70% by weight fibrous material            and from 30 to 60% by weight water-absorbent polymer            particles of the present invention;    -   (D) an optional acquisition-distribution layer between (A) and        (C), comprising        -   from 80 to 100% by weight fibrous material and from 0 to 20%            by weight water-absorbent polymer particles of the present            invention;        -   preferably from 85 to 99.9% by weight fibrous material and            from 0.01 to 15% by weight water-absorbent polymer particles            of the present invention;        -   more preferably from 90 to 99.5% by weight fibrous material            and from 0.5 to 10% by weight water-absorbent polymer            particles of the present invention;        -   most preferably from 95 to 99% by weight fibrous material            and from 1 to 5% by weight water-absorbent polymer particles            of the present invention;    -   (E) an optional tissue layer disposed immediately above and/or        below (C); and    -   (F) other optional components.

Fluid-absorbent articles are understood to mean, for example,incontinence pads and incontinence briefs for adults or diapers forbabies. Suitable fluid-absorbent articles including fluid-absorbentcompositions comprising fibrous materials and optionally water-absorbentpolymer particles to form fibrous webs or matrices for the substrates,layers, sheets and/or the fluid-absorbent core.

The acquisition-distribution layer acts as transport and distributionlayer of the discharged body fluids and is typically optimized to affectefficient liquid distribution with the underlying fluid-absorbent core.Hence, for quick temporary liquid retention it provides the necessaryvoid space while its area coverage of the underlying fluid-absorbentcore must affect the necessary liquid distribution and is adopted to theability of the fluid-absorbent core to quickly dewater theacquisition-distribution layer.

For fluid-absorbent articles that possess a very good dewatering thathas excellent wicking capability it is advantageous to useacquisition-distribution layers. For fluid-absorbent articles thatpossess a fluid-absorbent core comprising very permeable water-absorbentpolymer particles a small and thin acquisition-distribution layer can beused.

Suitable fluid-absorbent articles are composed of several layers whoseindividual elements must show preferably definite functional parametersuch as dryness for the upper liquid-pervious layer, vapor permeabilitywithout wetting through for the lower liquid-impervious layer, aflexible, vapor permeable and thin fluid-absorbent core, showing fastabsorption rates and being able to retain highest quantities of bodyfluids, and an acquisition-distribution layer between the upper layerand the core. These individual elements are combined such that theresultant fluid-absorbent article meets overall criteria such asflexibility, water vapor breathability, dryness, wearing comfort andprotection on the one side, and concerning liquid retention, rewet andprevention of wet through on the other side. The specific combination ofthese layers provides a fluid-absorbent article delivering both highprotection levels as well as high comfort to the consumer.

The products as obtained by the present invention are also very suitableto be incorporated into low-fluff, low-fiber, fluff-less, or fiber-lesshygiene article designs. Such designs and methods to make them are forexample described in the following publications and literature citedtherein and are expressly incorporated into the present invention: EP 2301 499 A1, EP 2 314 264 A1, EP 2 387 981 A1, EP 2 486 901 A1, EP 2 524679 A1, EP 2 524 679 A1, EP 2 524 680 A1, EP 2 565 031 A1, U.S. Pat. No.6,972,011, US 2011/0162989, US 2011/0270204, WO 2010/004894 A1, WO2010/004895 A1, WO 2010/076857 A1, WO 2010/082373 A1, WO 2010/118409 A1,WO 2010/133529 A2, WO 2010/143635 A1, WO 2011/084981 A1, WO 2011/086841A1, WO 2011/086842 A1, WO 2011/086843 A1, WO 2011/086844 A1, WO2011/117997 A1, WO 2011/136087 A1, WO 2012/048879 A1, WO 2012/052173 A1und WO 2012/052172 A1.

The present invention further provides fluid-absorbent articles,comprising water-absorbent polymer particles of the present inventionand less than 15% by weight fibrous material and/or adhesives in theabsorbent core.

The water-absorbent polymer particles and the fluid-absorbent articlesare tested by means of the test methods described below.

Methods:

The measurements should, unless stated otherwise, be carried out at anambient temperature of 23±2° C. and a relative atmospheric humidity of50±10%. The water-absorbent polymers are mixed thoroughly before themeasurement.

Vortex

50.0±1.0 ml of 0.9% NaCl solution are added into a 100 ml beaker. Acylindrical stirrer bar (30×6 mm) is added and the saline solution isstirred on a stir plate at 60 rpm. 2.000±0.010 g of water-absorbentpolymer particles are added to the beaker as quickly as possible,starting a stop watch as addition begins. The stopwatch is stopped whenthe surface of the mixture becomes ‘still’ that means the surface has noturbulence, and while the mixture may still turn, the entire surface ofparticles turns as a unit. The displayed time of the stopwatch isrecorded as Vortex time.

Residual Monomers

The level of residual monomers in the water-absorbent polymer particlesis determined by the EDANA recommended test method No. WSP 210.3-(11)“Residual Monomers”.

Particle Size Distribution

The particle size distribution of the water-absorbent polymer particlesis determined with the Camziser® image analysis system (RetschTechnology GmbH; Haan; Germany).

For determination of the average particle diameter and the particlediameter distribution the proportions of the particle fractions byvolume are plotted in cumulated form and the average particle diameteris determined graphically.

The average particle diameter (APD) here is the value of the mesh sizewhich gives rise to a cumulative 50% by weight.

The particle diameter distribution (PDD) is calculated as follows:

${{PDD} = \frac{x_{2} - x_{1}}{APD}},$wherein x₁ is the value of the mesh size which gives rise to acumulative 90% by weight and x₂ is the value of the mesh size whichgives rise to a cumulative 10% by weight.Mean Sphericity

The mean sphericity is determined with the Camziser® image analysissystem (Retsch Technology GmbH; Haan; Germany) using the particlediameter fraction from 100 to 1,000 μm.

Moisture Content

The moisture content of the water-absorbent polymer particles isdetermined by the EDANA recommended test method No. WSP 230.3 (11) “MassLoss Upon Heating”.

Centrifuge Retention Capacity (CRC)

The centrifuge retention capacity of the water-absorbent polymerparticles is determined by the EDANA recommended test method No. WSP241.3 (11) “Free Swell Capacity in Saline, After Centrifugation”,wherein for higher values of the centrifuge retention capacity largertea bags have to be used.

Absorbency Under No Load (AUNL)

The absorbency under no load of the water-absorbent polymer particles isdetermined analogously to the EDANA recommended test method No. WSP242.3 (11) “Gravimetric Determination of Absorption Under Pressure”,except using a weight of 0.0 g/cm² instead of a weight of 21.0 g/cm².

Absorbency Under Load (AUL)

The absorbency under load of the water-absorbent polymer particles isdetermined by the EDANA recommended test method No. WSP 242.3 (11)“Gravimetric Determination of Absorption Under Pressure”

Absorbency Under High Load (AUHL)

The absorbency under high load of the water-absorbent polymer particlesis determined analogously to the EDANA recommended test method No. WSP242.3 (11) “Gravimetric Determination of Absorption Under Pressure”,except using a weight of 49.2 g/cm² instead of a weight of 21.0 g/cm².

Porosity

The porosity of the water-absorbent polymer particles is calculated asfollows:

${Porosity} = \frac{{AUNL} - {CRC}}{AUNL}$Bulk Density/Flow Rate

The bulk density (BD) and the flow rate (FR) of the water-absorbentpolymer particles is determined by the EDANA recommended test method No.WSP 250.3 (11) “Gravimetric Determination of flow rate, GravimetricDetermination of Density”.

Extractables

The level of extractable constituents in the water-absorbent polymerparticles is determined by the EDANA recommended test method No. WSP470.2-05 “Extractables”.

Saline Flow Conductivity (SFC)

The saline flow conductivity is, as described in EP 0 640 330 A1,determined as the gel layer permeability of a swollen gel layer offluid-absorbent polymer particles, although the apparatus described onpage 19 and in FIG. 8 in the aforementioned patent application wasmodified to the effect that the glass frit (40) is no longer used, theplunger (39) consists of the same polymer material as the cylinder (37)and now comprises 21 bores having a diameter of 9.65 mm each distributeduniformly over the entire contact surface. The procedure and theevaluation of the measurement remains unchanged from EP 0 640 330 A1.The flow rate is recorded automatically.

The saline flow conductivity (SFC) is calculated as follows:SFC[cm³s/g]=(Fg(t=0)×L0)/(d×A×WP),where Fg(t=0) is the flow rate of NaCl solution in g/s, which isobtained by means of a linear regression analysis of the Fg(t) data ofthe flow determinations by extrapolation to t=0, L0 is the thickness ofthe gel layer in cm, d is the density of the NaCl solution in g/cm³, Ais the surface area of the gel layer in cm² and WP is the hydrostaticpressure over the gel layer in dyn/cm².Free Swell Rate (FSR)

1.00 g (=W1) of the dry fluid-absorbent polymer particles is weighedinto a 25 ml glass beaker and is uniformly distributed on the base ofthe glass beaker. 20 ml of a 0.9% by weight sodium chloride solution arethen dispensed into a second glass beaker, the content of this beaker israpidly added to the first beaker and a stopwatch is started. As soon asthe last drop of salt solution is absorbed, confirmed by thedisappearance of the reflection on the liquid surface, the stopwatch isstopped. The exact amount of liquid poured from the second beaker andabsorbed by the polymer in the first beaker is accurately determined byweighing back the second beaker (=W2). The time needed for theabsorption, which was measured with the stopwatch, is denoted t. Thedisappearance of the last drop of liquid on the surface is defined astime t.

The free swell rate (FSR) is calculated as follows:FSR[g/gs]=W2/(W1×t)

When the moisture content of the hydrogel-forming polymer is more than3% by weight, however, the weight W1 must be corrected for this moisturecontent.

Volumetric Absorbency Under Load (VAUL)

The volumetric absorbency under a load is used in order to measure theswelling kinetics, i.e. the characteristic swelling time, ofwater-absorbent polymer particles under different applied pressures. Theheight of swelling is recorded as a function of time.

The set up is show in FIG. 14 and consists of

-   -   An ultrasonic distance sensor (85) type BUS M18K0-XBFX-030-504K        (Balluff GmbH, Neuhausen a.d.F.; Germany) is placed above the        cell. The sensor receives ultrasound reflected by the metal        plate. The sensor is connected to an electronic recorder.    -   A PTFE cell (86) having a diameter of 75 mm, a height of 73 mm        and an internal diameter of 52 mm    -   A cylinder (87) made of metal or plastic having a diameter of 50        mm, a height of 71 mm and a mesh bottom)    -   A metal reflector (88) having a diameter of 57 mm and a height        of 45 mm    -   Metal ring weights (89) having a diameter of 100 mm and weights        calibrated to 278.0 g or 554.0 g

It is possible to adjust the pressure applied to the sample by changingthe combination of cylinder (86) and metal ring (88) weight assummarized in the following tables:

Available Equipment Weight psi Metal reflector  13.0 g 0.009 Plasticcylinder  28.0 g 0.020 Metal cylinder 126.0 g 0.091 Small ring weight278.0 g 0.201 Large ring weight 554.0 g 0.401

Possible Combinations psi Metal reflector + plastic cylinder 0.03 Metalreflector + metal cylinder 0.10 Metal reflector + metal cylinder + smallring weight 0.30 Metal reflector + metal cylinder + large ring weight0.50 Metal reflector + metal cylinder + small ring weight + 0.70 largering weight

A sample of 2.0 g of water-absorbent polymer particles is placed in thePTFE cell (86). The cylinder (equipped with mesh bottom) and the metalreflector (88) on top of it are placed into the PTFE cell (86). In orderto apply higher pressure, metal rings weights (89) can be placed on thecylinder.

60.0 g of aqueous saline solution (0.9% by weight) are added into thePTFE cell (86) with a syringe and the recording is started. During theswelling, the water-absorbent polymer particles push the cylinder (87)up and the changes in the distance between the metal reflector (88) andthe sensor (85) are recorded.

After 120 minutes, the experiment is stopped and the recorded data aretransferred from the recorder to a PC using a USB stick. Thecharacteristic swelling time is calculated according to the equationQ(t)=Q_(max)·(1−e^(−t/τ)) as described by “Modern Superabsorbent PolymerTechnology” (page 155, equation 4.13) wherein Q(t) is the swelling ofthe superabsorbent which is monitored during the experiment, Q_(max)corresponds to the maximum swelling reached after 120 minutes (end ofthe experiment) and τ is the characteristic swelling time (τ is theinverse rate constant k).

Using the add-in functionality ‘Solver’ of Microsoft Excel software, atheoretical curve can be fitted to the measured data and thecharacteristic time for 0.03 psi is calculated.

The measurements are repeated for different pressures (0.1 psi, 0.3 psi,0.5 psi and 0.7 psi) using combinations of cylinder and ring weights.The characteristic swelling times for the different pressures can becalculated using the equation Q(t)=Q_(max)·(1−e^(−t/τ)).

Wicking Absorption

The wicking absorption is used in order to measure the total liquiduptake of water-absorbent polymer particles under applied pressure. Theset-up is show in FIG. 15.

A 500 ml glass bottle (90) (scale division 100 mL, height 26.5 cm)equipped with an exit tube of Duran® glass, is filled with 500 mL ofaqueous saline solution (0.9% by weight). The bottle has an opening atthe bottom end which can be connected to the Plexiglas plate through aflexible hose (91).

A balance (92) connected to a computer is placed on Plexiglas block(area 20×26 cm², height 6 cm). The glass bottle is then placed on thebalance.

A Plexiglas plate (93) (area: 11×11 cm², height: 3.5 cm) is placed on alifting platform. A porosity P1 glass frit of 7 cm in diameter and 0.45cm high (94) has been liquid-tightly embedded in the Plexiglas plate,i.e. the fluid exits through the pores of the frit and not via the edgebetween Plexiglas plate and frit. A Plexiglas tube leads through theouter shell of Plexiglas plate into the center of the Plexiglas plate upto the frit to ensure fluid transportation. The fluid tube is thenconnected with the flexible hose (35 cm in length, 1.0 cm externaldiameter, 0.7 cm internal diameter) to the glass bottle (90).

The lifting platform is used to adjust the upper side of the frit to thelevel of the bottom end of the glass bottle, so that an alwaysatmospheric flux of fluid from the bottle to the measuring apparatus isensured during measurement. The upper side of the frit is adjusted suchthat its surface is moist but there is no supernatant film of water onthe frit.

The fluid in the glass bottle (90) is made up to 500 ml before everyrun.

In a Plexiglas cylinder (95) (7 cm in external diameter, 6 cm ininternal diameter, 16 cm in height) and equipped with a 400 mesh (36 μm)at the bottom are placed 26 g of water-absorbent polymer particles. Thesurface of the water-absorbent polymer particles is smoothed. The filllevel is about 1.5 cm. Then a weight (96) of 0.3 psi (21.0 g/cm²) isplaced on top of the water-absorbent polymer particles.

The Plexiglas cylinder is placed on the (moist) frit and the electronicdata recording started. A decrease in the weight of the balance isregistered as a function of time. This then indicates how much aqueoussaline solution has been absorbed by the swelling gel of water-absorbentpolymer particles at a certain time. The data are automatically capturedevery 10 seconds. The measurement is carried out at 0.3 psi (21.0 g/cm²)for a period of 120 minutes per sample. The total liquid uptake is thetotal amount of aqueous saline solution absorbed by each 26 g sample.

Rewet Under Load (RUL)

The test determines the amount of fluid a fluid-absorbent article willrelease after being maintained at a pressure of 0.7 psi (49.2 g/cm²) for10 min following multiple separate insults. The rewet under load ismeasured by the amount of fluid the fluid-absorbent article releasesunder pressure. The rewet under load is measured after each insult.

The fluid-absorbent article is clamped nonwoven side upward onto theinspection table. The insult point is marked accordingly with regard tothe type and gender of the diaper to be tested (i.e. in the centre ofthe core for girl, 2.5 cm towards the front for unisex and 5 cm towardsthe front for boy). A 3.64 kg circular weight (10 cm diameter) having acentral opening (2.3 cm diameter) with perspex tube is placed with onthe previously marked insult point.

For the primary insult 100 g of aqueous saline solution (0.9% by weight)is poured into the perspex tube in one shot. Amount of time needed forthe fluid to be fully absorbed into the fluid-absorbent article isrecorded. After 10 minutes have elapsed, the load is removed and thestack of 10 filter papers (Whatman®) having 9 cm diameter and known dryweight (W1) is placed over the insult point on the fluid-absorbentarticle. On top of the filter paper, the 2.5 kg weight with 8 cmdiameter is added. After 2 minutes have elapsed the weight is removedand filter paper reweighed giving the wet weight value (W2).

The rewet under load is calculated as follows:RUL [g]=W2−W1

For the rewet under load of the secondary insult the procedure for theprimary insult is repeated. 50 g of aqueous saline solution (0.9% byweight) and 20 filter papers are used.

For the rewet under load of the tertiary and following insults theprocedure for the primary insult is repeated. For each of the followinginsults 3^(rd), 4^(th) and 5^(th) 50 g of aqueous saline solution (0.9%by weight) and 30, 40 and 50 filter papers respectively are used.

Rewet Value (RV)

This test consists of multiple insults of aqueous saline solution (0.9%by weight). The rewet value is measured by the amount of fluid thefluid-absorbent article released under pressure. The rewet is measuredafter each insult.

The fluid-absorbent article is clamped nonwoven side upward onto theinspection table. The insult point is marked accordingly with regard tothe type and gender of the diaper to be tested (i.e. in the centre ofthe core for girl, 2.5 cm towards the front for unisex and 5 cm towardsthe front for boy). A separatory funnel is positioned above thefluid-absorbent article so that the spout is directly above the markedinsult point.

For the primary insult 100 g of aqueous saline solution (0.9% by weight)is poured into the fluid-absorbent article via the funnel in one shot.The liquid is allowed to be absorbed for 10 minutes, and after that timethe stack of 10 filter papers (Whatman®) having 9 cm diameter and knowndry weight (D1) is placed over the insult point on the fluid-absorbentarticle. On top of the filter paper, the 2.5 kg weight with 8 cmdiameter is added. After 2 minutes have elapsed the weight is removedand filter paper reweighed giving the wet weight value (D2).

The rewet value is calculated as follows:RV [g]=D2−D1

For the rewet of the secondary insult the procedure for the primaryinsult is repeated. 50 g of aqueous saline solution (0.9% by weight) and20 filter papers are used.

For the rewet of the tertiary and following insults the procedure forthe primary insult is repeated. For each of the following insults3^(rd), 4^(th) and 5^(th) 50 g of aqueous saline solution (0.9% byweight) and 30, 40 and 50 filter papers respectively are used.

The EDANA test methods are obtainable, for example, from the EDANA,Avenue Eugène Plasky 157, B-1030 Brussels, Belgium.

EXAMPLES Preparation of the Base Polymer Example 1

The process was performed in a concurrent spray drying plant with anintegrated fluidized bed (27) and an external fluidized bed (29) asshown in FIG. 1. The cylindrical part of the spray dryer (5) had aheight of 22 m and a diameter of 3.4 m. The internal fluidized bed (IFB)had a diameter of 3 m and a weir height of 0.25 m. The externalfluidized bed (EFB) had a length of 3.0 m, a width of 0.65 m and a weirheight of 0.5 m.

The drying gas was fed via a gas distributor (3) at the top of the spraydryer. The drying gas was partly recycled (drying gas loop) via abaghouse filter (9) and a condenser column (12). The drying gas wasnitrogen that comprises from 1% to 4% by volume of residual oxygen:Before start of polymerization the drying gas loop was filled withnitrogen until the residual oxygen was below 4% by volume. The gasvelocity of the drying gas in the cylindrical part of the spray dryer(5) was 0.8 m/s. The pressure inside the spray dryer was 4 mbar belowambient pressure.

The spray dryer outlet temperature was measured at three points aroundthe circumference at the end of the cylindrical part as shown in FIG. 3.Three single measurements (47) were used to calculate the averagecylindrical spray dryer outlet temperature. The drying gas loop washeated up and the dosage of monomer solution is started up. From thistime the spray dryer outlet temperature was controlled to 117° C. byadjusting the gas inlet temperature via the heat exchanger (20).

The product accumulated in the internal fluidized bed (27) until theweir height was reached. Conditioned internal fluidized bed gas having atemperature of 122° C. and a relative humidity of 4% was fed to theinternal fluidized bed (27) via line (25). The gas velocity of theinternal fluidized bed gas in the internal fluidized bed (27) was 0.80m/s. The residence time of the product was 120 min.

The spray dryer offgas was filtered in baghouse filter (9) and sent to acondenser column (12) for quenching/cooling. Excess water was pumped outof the condenser column (12) by controlling the (constant) filling levelinside the condenser column (12). The water inside the condenser column(12) was cooled by a heat exchanger (13) and pumped counter-current tothe gas via quench nozzles (11) so that the temperature inside thecondenser column (12) was 45° C. The water inside the condenser column(12) was set to an alkaline pH by dosing sodium hydroxide solution towash out acrylic acid vapors.

The condenser column offgas was split to the drying gas inlet pipe (1)and the conditioned internal fluidized bed gas (25). The gastemperatures were controlled via heat exchangers (20) and (22). The hotdrying gas was fed to the concurrent spray dryer via gas distributor(3). The gas distributor (3) consists of a set of plates providing apressure drop of 2 to 4 mbar depending on the drying gas amount.

The product was discharged from the internal fluidized bed (27) viarotary valve (28) into external fluidized bed (29). Conditioned externalfluidized bed gas having a temperature of 60° C. was fed to the externalfluidized bed (29) via line (40). The external fluidized bed gas wasair. The gas velocity of the external fluidized bed gas in the externalfluidized bed (29) was 0.8 m/s. The residence time of the product was 1min.

The product was discharged from the external fluidized bed (29) viarotary valve (32) into sieve (32). The sieve (33) was used for sievingoff overs/lumps having a particle diameter of more than 800 μm.

The monomer solution was prepared by mixing first acrylic acid with3-tuply ethoxylated glycerol triacrylate (internal crosslinker) andsecondly with 37.3% by weight sodium acrylate solution. The temperatureof the resulting monomer solution was controlled to 10° C. by using aheat exchanger and pumping in a loop. A filter unit having a mesh sizeof 250 μm was used in the loop after the pump. The initiators weremetered into the monomer solution upstream of the dropletizer by meansof static mixers (41) and (42) via lines (43) and (44) as shown inFIG. 1. Sodium peroxodisulfate solution having a temperature of 20° C.was added via line (43) and[2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride solutiontogether with Bruggolite FF7 having a temperature of 5° C. was added vialine (44). Each initiator was pumped in a loop and dosed via controlvalves to each dropletizer unit. A second filter unit having a mesh sizeof 140 μm was used after the static mixer (42). For dosing the monomersolution into the top of the spray dryer three dropletizer units wereused as shown in FIG. 4.

A dropletizer unit consisted of an outer pipe (51) having an opening forthe dropletizer cassette (53) as shown in FIG. 5. The dropletizercassette (53) was connected with an inner pipe (52). The inner pipe (53)having a PTFE block (54) at the end as sealing can be pushed in and outof the outer pipe (51) during operation of the process for maintenancepurposes.

The temperature of the dropletizer cassette (61) was controlled to 8° C.by water in flow channels (59) as shown in FIG. 6. The dropletizercassette (61) had 256 bores having a diameter of 170 μm and a boreseparation of 15 mm. The dropletizer cassette (61) consisted of a flowchannel (60) having essential no stagnant volume for homogeneousdistribution of the premixed monomer and initiator solutions and onedroplet plate (57). The droplet plate (57) had an angled configurationwith an angle of 3°. The droplet plate (57) was made of stainless steeland had a length of 630 mm, a width of 128 mm and a thickness of 1 mm.

The feed to the spray dryer consisted of 10.45% by weight of acrylicacid, 33.40% by weight of sodium acrylate, 0.018% by weight of 3-tuplyethoxylated glycerol triacrylate, 0.072% by weight of[2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, 0.0029% byweight of Bruggolite FF7 (5% by weight in water), 0.054% by weight ofsodiumperoxodisulfate solution (15% by weight in water) and water. Thedegree of neutralization was 71%. The feed per bore was 1.6 kg/h.

The polymer particles (base polymer A1) exhibit the following featuresand absorption profile:

CRC of 40.2 g/g

AUNL of 51.8 g/g

AUL of 22.4 g/g

AUHL of 8.2 g/g

Porosity of 22.3%

Extractables of 4.3 wt. %

Residual monomers of 12161 ppm

Moisture content of 6.1 wt. %

Vortex time of 67 s

The resulting polymer particles had a bulk density of 68 g/100 ml and anaverage particle diameter of 407 μm.

Example 2

Example 1 was repeated, except that the resulting polymer particleshaving a content of the residual monomer of 12161 ppm were demonomerizedin a plastic bottle in the lab oven at 90° C. for 60 minutes afterspraying 15% by weight of water onto the polymer particles in alaboratory ploughshare mixer. Therefore the content of the residualmonomer was decreased to 256 ppm and the moisture content was increasedto 17.5% by weight.

The polymer particles (base polymer B1) exhibit the following featuresand absorption profile:

CRC of 33.1 g/g

AUNL of 42.3 g/g

AUL of 17.0 g/g

AUHL of 8.1 g/g

Porosity of 21.7%

Extractables of 8.2 wt. %

Residual monomers of 256 ppm

Moisture content of 17.5 wt. %

Vortex time of 54 s

Example 3

Example 1 was repeated, except that the feed to the spray dryerconsisted of 0.036% by weight of[2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride and that theconditioned internal fluidized bed gas had a temperature of 122° C. anda relative humidity of 4% and that the residence time of the product inthe internal fluidized bed was 120 min.

The polymer particles (base polymer C1) exhibit the following featuresand absorption profile:

CRC of 39.5 g/g

AUNL of 51.4 g/g

AUHL of 9.0 g/g

Porosity of 23.2%

Residual monomers of 1581 ppm

Moisture content of 10.9 wt. %

Surface-Postcrosslinking of the Base Polymer Example 4

1200 g of the water-absorbent polymer particles prepared in Example 1(base polymer A1) having a content of residual monomers of 12161 ppmwere put into a laboratory ploughshare mixer (model MR5, manufactured byGebrüder Lödige Maschinenbau GmbH; Paderborn; Germany). Asurface-postcrosslinker solution was prepared by mixing 60 g ofsurface-postcrosslinker as described in Table 1 and 60 g of deionizedwater, into a beaker. At a mixer speed of 200 rpm, the aqueous solutionwas sprayed onto the polymer particles within one minute by means of aspray nozzle. The mixing was continued for additional 5 minutes. Theproduct was removed and transferred into another ploughshare mixer(model MR5, manufactured by Gebrüder Lödige Maschinenbau GmbH;Paderborn; Germany) which was heated to 140° C. before. After mixing forfurther 80 minutes at 140° C. with sample taking every 10 minutes, theproduct was removed from the mixer and sifted from 150 to 850 μm. Thesamples were analyzed. The results are summarized in Table 1.

The resulting polymer particles that were surface-postcrosslinked withethylene carbonate had a bulk density of 69.0 g/100 ml, an averageparticle diameter (APD) of 481 μm, a particle diameter distribution(PDD) of 0.28, and a mean sphericity of 0.82.

TABLE 1 Effect of the postcrosslinking agent Curing Residual Postcross-Time Monomers Moisture CRC AUNL AUL AUHL Porosity Extractables Vortexlinker [min] [ppm] [wt. %] [g/g] [g/g] [g/g] [g/g] [%] [wt. %] [s] EC 10533 10.1 49.2 58.1 18.8 8.2 15.3 5.3 78 20 384 7.0 51.4 60.8 20.6 8.015.5 4.5 77 30 376 5.1 51.6 61.0 28.0 9.5 15.4 4.8 76 40 386 3.6 50.562.1 31.9 12.9 18.7 3.8 74 50 432 1.9 49.1 61.8 36.2 18.2 20.6 3.4 70 60416 1.1 47.3 61.2 37.9 22.2 22.7 3.1 75 70 417 0.6 46.7 62.5 38.6 24.325.3 3.1 75 80 429 0.4 46.2 62.6 39.5 24.2 26.2 3.1 75 HEONON 10 15387.6 50.2 58.4 16.6 7.7 14.0 7.0 86 20 1377 5.3 51.3 59.7 16.3 7.8 14.16.8 85 30 1153 3.7 52.5 61.3 17.7 7.9 14.4 7.1 84 40 1061 2.8 51.5 61.419.8 8.0 16.1 6.6 86 50 1002 2.5 50.9 60.3 25.2 8.7 15.6 6.3 85 60 9972.4 49.1 59.9 27.7 10.3 18.0 6.0 82 70 939 2.2 48.1 59.8 30.0 13.1 19.65.8 81 80 913 2.0 47.3 59.3 32.6 16.2 20.2 5.1 81 EGDGE 10 714 8.0 34.243.7 23.1 17.1 21.7 22.1 95 20 572 5.4 35.2 44.7 23.6 17.2 21.3 23.3 9330 554 3.9 35.9 45.0 24.2 17.8 20.2 23.8 91 40 549 2.8 36.3 45.5 24.418.0 20.2 24.2 94 50 544 2.2 36.6 46.2 24.5 18.3 20.8 23.5 96 60 550 1.936.6 45.9 23.8 18.4 20.3 23.8 93 70 542 1.6 36.5 46.3 23.8 18.4 21.223.9 94 80 562 1.4 37.1 46.4 24.0 18.5 20.0 24.1 92 EC: Ethylenecarbonate; HEONON: N-(2-hydroxy ethyl)-2-oxazolidinone; EGDGE: Ethyleneglycol diglycidyl ether

Example 5

1200 g of the water-absorbent polymer particles as described in Table 2having different contents of residual monomers were put into alaboratory ploughshare mixer (model MR5, manufactured by Gebrüder LödigeMaschinenbau GmbH; Paderborn; Germany). A surface-postcrosslinkersolution was prepared by mixing 30 g ethylene carbonate and 30 g ofdeionized water, into a beaker. At a mixer speed of 200 rpm, the aqueoussolution was sprayed onto the polymer particles within one minute bymeans of a spray nozzle. The mixing was continued for additional 5minutes. The product was removed and transferred into anotherploughshare mixer (model MR5, manufactured by Gebrüder LödigeMaschinenbau GmbH; Paderborn; Germany) which was heated to 150° C.before. After mixing for further 80 minutes at 150° C. with sampletaking every 10 minutes, the product was removed from the mixer andsifted from 150 to 850 μm. The samples were analyzed. The results aresummarized in Table 2.

The resulting polymer particles based on base polymer A1 had a bulkdensity of 68.0 g/100 ml, an average particle diameter (APD) of 397 μm,a particle diameter distribution (PDD) of 0.38, and a mean sphericity of0.87.

The resulting polymer particles based on base polymer B1 had a bulkdensity of 64.7 g/100 ml, an average particle diameter (APD) of 553 μm,a particle diameter distribution (PDD) of 0.25, and a mean sphericity of0.74.

The resulting polymer particles based on base polymer C1 had a bulkdensity of 69.8 g/100 ml, an average particle diameter (APD) of 377 μm,a particle diameter distribution (PDD) of 0.42, and a mean sphericity of0.86.

Example 6

1200 g of the water-absorbent polymer particles as described in Table 3having different contents of residual monomers were put into alaboratory ploughshare mixer (model MR5, manufactured by Gebrüder LödigeMaschinenbau GmbH; Paderborn; Germany). A surface-postcrosslinkersolution was prepared by mixing 30 g ethylene carbonate and 60 g ofdeionized water, into a beaker. At a mixer speed of 200 rpm, the aqueoussolution was sprayed onto the polymer particles within one minute bymeans of a spray nozzle. The mixing was continued for additional 5minutes. The product was removed and transferred into anotherploughshare mixer (model MR5, manufactured by Gebrüder LödigeMaschinenbau GmbH; Paderborn; Germany) which was heated to 140° C.before. After mixing for further 80 minutes at 140° C. with sampletaking every 10 minutes, the product was removed from the mixer andsifted from 150 to 850 μm. The samples were analyzed. The results aresummarized in Table 3.

The resulting polymer particles based on base polymer A1 had a bulkdensity of 65.6 g/100 ml, an average particle diameter (APD) of 450 μm,a particle diameter distribution (PDD) of 0.32, and a mean sphericity of0.82.

The resulting polymer particles based on base polymer B1 had a bulkdensity of 64.7 g/100 ml, an average particle diameter (APD) of 564 μm,a particle diameter distribution (PDD) of 0.22, and a mean sphericity of0.75.

The resulting polymer particles based on base polymer C1 had a bulkdensity of 70.3 g/100 ml, an average particle diameter (APD) of 399 μm,a particle diameter distribution (PDD) of 0.36, and a mean sphericity of0.84.

TABLE 2 Effect of the residual monomers Curing Residual Base TimeMonomers Moisture CRC AUNL AUL AUHL Porosity Extractables Vortex Polymer[min] [ppm] [wt. %] [g/g] [g/g] [g/g] [g/g] [%] [wt. %] [s] Base 10 21875.2 57.5 61.4 11.0 7.6 6.4 8.4 Polymer A1 20 1646 3.0 57.7 63.8 12.6 8.29.6 7.2 30 1273 1.5 53.5 65.4 33.5 11.6 18.2 5.3 40 1229 0.5 51.5 63.938.6 18.8 19.4 4.7 50 1225 0.5 49.7 63.5 40.6 24.5 21.7 5.0 60 1214 0.347.6 63.5 40.8 27.6 25.0 6.1 70 70 1243 0.3 48.1 62.7 40.3 30.2 23.3 6.269 80 1225 0.1 46.4 60.1 38.2 30.7 22.8 6.3 67 Base 10 133 13.7 35.8Polymer B1 20 133 8.4 36.9 30 136 3.3 35.5 89 40 157 1.9 33.2 7.5 93 50160 1.3 30.9 43.0 27.7 18.9 28.1 92 60 166 0.9 28.8 40.9 27.3 20.3 29.695 70 172 0.7 26.7 38.2 26.6 21.2 30.1 94 80 178 0.7 25.9 36.9 26.4 21.229.8 3.7 94 Base 10 399 8.0 42.2 53.8 28.1 10.7 21.6 70 Polymer C1 20398 3.8 43.3 56.6 32.1 13.6 23.5 70 30 417 1.8 44.1 56.9 36.4 21.7 22.571 40 446 1.3 43.0 55.9 38.4 25.4 23.1 3.1 73 50 419 1.0 39.5 54.3 36.929.0 27.3 73 60 413 0.8 38.7 52.1 37.0 29.5 25.7 75 70 403 0.6 37.6 51.636.2 29.4 27.1 76 80 402 0.6 36.4 51.3 35.0 29.7 29.0 3.3 76

TABLE 3 Effect of the residual monomers Curing Residual Base TimeMonomers Moisture CRC AUNL AUL AUHL Porosity Extractables Vortex Polymer[min] [ppm] [wt. %] [g/g] [g/g] [g/g] [g/g] [%] [wt. %] [s] Base 10 5067.7 51.7 59.3 17.4 7.8 12.8 5.9 Polymer A1 20 417 4.9 53.0 61.2 22.1 8.013.4 5.5 30 312 2.7 51.6 61.9 29.8 10.8 16.6 4.8 40 328 2.0 52.2 62.335.5 14.6 16.2 4.1 50 359 1.1 50.1 62.1 37.3 19.0 19.3 4.3 60 370 0.849.5 62.9 38.4 22.6 21.3 4.3 70 385 0.7 47.1 61.9 39.0 25.4 23.9 4.2 7380 340 0.5 45.3 60.3 37.9 27.5 24.9 4.0 72 Base 10 33.7 Polymer B1 200.8 36.1 8.1 30 36.1 8.6 40 123 4.1 35.4 48.1 22.9 10.2 26.4 50 124 2.833.8 46.5 25.8 11.7 27.3 87 60 125 2.3 32.6 45.0 25.4 13.1 27.6 88 70132 0.9 31.9 44.8 26.1 14.4 28.8 92 80 138 0.8 31.2 43.8 26.4 13.7 28.890 Base 10 373 9.6 41.1 52.9 28.8 13.1 22.0 70 Polymer C1 20 356 5.642.6 55.1 31.5 14.4 23.0 71 30 362 3.0 42.0 55.6 34.1 19.6 24.0 72 40378 1.3 42.2 55.8 34.8 23.2 24.0 4.2 73 50 381 1.1 41.4 55.2 34.7 24.925.0 73 60 389 0.7 40.2 55.4 35.2 26.0 27.0 72 70 396 0.6 40.9 53.9 34.426.8 24.0 70 80 395 0.4 40.6 53.5 35.2 26.6 24.0 4.2 72

Example 7

The process was performed in a concurrent spray drying plant with anintegrated fluidized bed (27) and an external fluidized bed (29) asshown in FIG. 1. The cylindrical part of the spray dryer (5) had aheight of 22 m and a diameter of 3.4 m. The internal fluidized bed (IFB)had a diameter of 3 m and a weir height of 0.25 m.

The drying gas was fed via a gas distributor (3) at the top of the spraydryer. The drying gas was partly recycled (drying gas loop) via abaghouse filter (9) and a condenser column (12). Instead of the baghousefilter (9) any other filter and/or cyclone can be used. The drying gaswas nitrogen that comprises from 1% to 4% by volume of residual oxygen:Before start of polymerization the drying gas loop was filled withnitrogen until the residual oxygen was below 4% by volume. The gasvelocity of the drying gas in the cylindrical part of the spray dryer(5) was 0.82 m/s. The pressure inside the spray dryer was 4 mbar belowambient pressure.

The spray dryer outlet temperature was measured at three points aroundthe circumference at the end of the cylindrical part as shown in FIG. 3.Three single measurements (47) were used to calculate the averagecylindrical spray dryer outlet temperature. The drying gas loop washeated up and the dosage of monomer solution is started up. From thistime the spray dryer outlet temperature was controlled to 118° C. byadjusting the gas inlet temperature via the heat exchanger (20). The gasinlet temperature was 167° C. and the steam content of the drying gaswas 0.058 kg steam per kg dry gas.

The product accumulated in the internal fluidized bed (27) until theweir height was reached. Conditioned internal fluidized bed gas having atemperature of 104° C. and a steam content of 0.058 or 0.130 kg steamper kg dry gas was fed to the internal fluidized bed (27) via line (25).The gas velocity of the internal fluidized bed gas in the internalfluidized bed (27) was 0.65 m/s. The residence time of the product was150 min. The temperature of the water-absorbent polymer particles in theinternal fluidized bed was 82° C.

The spray dryer offgas was filtered in baghouse filter (9) and sent to acondenser column (12) for quenching/cooling. Excess water was pumped outof the condenser column (12) by controlling the (constant) filling levelinside the condenser column (12). The water inside the condenser column(12) was cooled by a heat exchanger (13) and pumped counter-current tothe gas via quench nozzles (11) so that the temperature inside thecondenser column (12) was 45° C. The water inside the condenser column(12) was set to an alkaline pH by dosing sodium hydroxide solution towash out acrylic acid vapors.

The condenser column offgas was split to the drying gas inlet pipe (1)and the conditioned internal fluidized bed gas (25). The gastemperatures were controlled via heat exchangers (20) and (22). The hotdrying gas was fed to the concurrent spray dryer via gas distributor(3). The gas distributor (3) consists of a set of plates providing apressure drop of 2 to 4 mbar depending on the drying gas amount.

The product was discharged from the internal fluidized bed (27) viarotary valve (28) into sieve (29). The sieve (29) was used for sievingoff overs/lumps having a particle diameter of more than 800 μm.

The monomer solution was prepared by mixing first acrylic acid with3-tuply ethoxylated glycerol triacrylate (internal crosslinker) andsecondly with 37.3% by weight sodium acrylate solution. The temperatureof the resulting monomer solution was controlled to 10° C. by using aheat exchanger and pumping in a loop. A filter unit having a mesh sizeof 250 μm was used in the loop after the pump. The initiators weremetered into the monomer solution upstream of the dropletizer by meansof static mixers (41) and (42) via lines (43) and (44) as shown inFIG. 1. Sodium peroxodisulfate solution having a temperature of 20° C.was added via line (43) and[2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride solutiontogether with Bruggolite FF7 having a temperature of 5° C. was added vialine (44). Each initiator was pumped in a loop and dosed via controlvalves to each dropletizer unit A second filter unit having a mesh sizeof 140 μm was used after the static mixer (42). For dosing the monomersolution into the top of the spray dryer three dropletizer units wereused as shown in FIG. 4.

A dropletizer unit consisted of an outer pipe (51) having an opening forthe dropletizer cassette (53) as shown in FIG. 5. The dropletizercassette (53) was connected with an inner pipe (52). The inner pipe (53)having a PTFE block (54) at the end as sealing can be pushed in and outof the outer pipe (51) during operation of the process for maintenancepurposes.

The temperature of the dropletizer cassette (61) was controlled to 8° C.by water in flow channels (59) as shown in FIG. 6. The dropletizercassette (61) had 256 bores having a diameter of 170 μm and a boreseparation of 15 mm. The dropletizer cassette (61) consisted of a flowchannel (60) having essential no stagnant volume for homogeneousdistribution of the premixed monomer and initiator solutions and onedroplet plate (57). The droplet plate (57) had an angled configurationwith an angle of 3°. The droplet plate (57) was made of stainless steeland had a length of 630 mm, a width of 128 mm and a thickness of 1 mm.

The feed to the spray dryer consisted of 10.45% by weight of acrylicacid, 33.40% by weight of sodium acrylate, 0.018% by weight of 3-tuplyethoxylated glycerol triacrylate, 0.036% by weight of[2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, 0.0029% byweight of Bruggolite FF7, 0.054% by weight of sodium peroxodisulfate andwater. The degree of neutralization was 71%. The feed per bore was 1.4kg/h.

The resulting water-absorbent polymer particles were analyzed. Theresults are summarized in Table 4.

TABLE 4 Base polymers, used for the surface-postcrosslinking reactionsResidual Steam Content Bulk Density CRC Monomers Extractables MoistureFSR Example [kg/kg] [g/cm³] [g/g] [ppm] [wt. %] [wt. %] [g/gs] 7a 0.0580.74 47.2 9300 4.5 5.7 0.20 7b 0.130 0.71 49.1 4000 5.4 7.5 0.07

Examples 8 to 26

In a Schugi Flexomix® (model Flexomix-160, manufactured by HosokawaMicron B.V., Doetinchem, the Netherlands) with a speed of 2000 rpm, thebase polymer 7a or 7b was coated with a surface-postcrosslinker solutionby using 2 or 3 round spray nozzle systems (model Gravity-Fed SpraySet-ups, External Mix Typ SU4, Fluid Cap 60100 and Air Cap SS-120,manufactured by Spraying Systems Co, Wheaton, Ill., USA) and then filledvia inlet (74) and dried in a NARA heater (model NPD SW-18, manufacturedby GMF Gouda, Waddinxveen, the Netherlands) with a speed of the shaft(80) of 6 rpm. The NARA heater has two paddles with a shaft offset of90° (84) and a fixed discharge zone (75) with two flexible weir plates(77). Each weir has a weir opening with a minimal weir height at 50%(79) and a maximal weir opening at 100% (78) as shown in FIG. 13.

The inclination angle α (82) between the floor plate and the NARA paddledryer is approx. 3°. The weir height of the NARA heater is between 50 to100% corresponding to a residence time of approx. 40 to 150 min, by aproduct density of approx. 700 to 750 kg/m³. The product temperature inthe NARA heater is in a range of 120 to 165° C. After drying, thesurface-postcrosslinked base polymer was transported over discharge cone(81) in the NARA cooler (GMF Gouda, Waddinxveen, the Netherlands), tocool down the surface postcrosslinked base polymer to approx. 60° C.with a speed of 11 rpm and a weir height of 145 mm. After cooling, thematerial was sieved with a minimum cut size of 150 μm and a maximum sizecut of 710 μm.

Ethylene carbonate, water, Plantacare® UP 818 (BASF SE, Ludwigshafen,Germany) and aqueous aluminum lactate (26% by weight) was premixed andspray coated as summarized in Tab 6. Aqueous aluminum sulfate (26% byweight) was separate spray coated (position of the nozzle=180°). Asaluminum lactate, Lothragon® Al 220 (manufactured by Dr. Paul LohmannGmbH, Emmerthal, Germany) was used.

The metered amounts and conditions of the coating into the SchugiFlexomix®, the conditions, the formulation and values of the drying andcooling step are summarized in Table 5 to 6:

All physical properties of the resulting polymers are summarized inTable 7 and 8:

TABLE 5 Process parameters of the thermal treatment in the heaterProduct Temp. Steam Steam Set Pressure Pressure Heater Heater HeaterHeater Heater Heater Through- Heater Value valve Jacket T1 T2 T3 T4 T5T6 put Weir No. of Pos. of Example ° C. bar bar ° C. ° C ° C. ° C. ° C.° C. kg/h % Nozzles Nozzles Polymer particles without aluminum salt 8140 4.6 4.3 84 81 111 123 130 140 400 56 2 90/270° 9 150 6.2 6.2 90 86115 129 137 150 400 56 2 90/270° 10 150 7.4 7.4 79 81 110 121 133 159500 67 3 90/180/270° Polymer particles with aluminum lactate 11 120 2.52.3 69 72 103 111 114 120 500 57 3 90/180/270° 12 130 2.3 3.5 75 84 110117 121 130 500 57 3 90/180/270° 13 140 6.7 6.6 82 93 118 127 135 150500 67 3 90/180/270° 14 150 5.5 5.0 84 107 117 131 138 150 400 56 290/270° 15 150 5.2 6.2 71 100 118 132 140 150 400 56 2 90/270° 16 1505.9 5.9 91 109 120 131 140 150 500 68 3 90/180/270° 17 150 6.1 6.1 83110 120 131 139 150 500 68 3 90/180/270° 18 160 6.5 6.5 89 114 123 138151 160 400 82 3 90/270° 19 170 8.1 8.1 91 113 129 151 163 170 400 82 290/270° Polymer particles with aluminum sulfate 20 150 5.6 5.5 66 99 119137 145 150 500 87 3 90/180/270° 21 150 5.0 5.0 95 96 121 135 144 150400 75 3 90/180/270° 22 150 5.6 5.6 74 104 117 127 136 150 500 87 390/180/270° 23 155 6.1 6.0 79 100 119 130 142 155 500 87 3 90/180/270°24 160 6.6 6.6 109 115 124 143 154 160 400 75 3 90/180/270° 25 160 6.56.5 96 105 125 144 154 160 400 75 3 90/180/270° 26 165 7.8 7.8 79 109122 138 152 165 500 87 3 90/180/270°

TABLE 6 Surface-postcrosslinker formulation of the thermal treatment inthe heater Al- Al- Plantacare lactate sulfate Base EC Water 818 UP (dry)(dry) Example polymer bop % bop % bop ppm bop % bop % Polymer particleswithout aluminum salt 8 7b 2.5 5.0 50 9 7b 2.5 5.0 50 10 7b 2.5 5.0 25Polymer particles with aluminum lactate 11 7b 2.5 5.0 25 0.5 12 7b 2.55.0 25 0.5 13 7b 1.5 5.0 50 0.5 14 7a 2.5 5.0 0.5 15 7a 2.5 5.0 50 0.516 7a 2.5 5.0 25 0.5 17 7b 2.5 5.0 25 0.5 18 7b 2.5 5.0 25 0.5 19 7a 2.55.0 25 0.5 Polymer particles with aluminum sulfate 20 7b 2.5 5.0 25 0.5021 7a 2.5 5.0 25 0.36 22 7b 2.5 5.0 25 0.75 23 7b 2.5 5.0 25 0.50 24 7b2.5 5.0 50 0.36 25 7a 2.5 5.0 25 0.36 26 7b 2.5 5.0 25 0.50 EC: Ethylenecarbonate; bop: based on polymer

TABLE 7 Physical properties of the polymer particles aftersurface-postcrosslinking Resid- ual Ex- Bulk SFC Vor- Mois- Mono- tract-Den- Fines <150 Overs >710 Exam- CRC AUNL AUL AUHL 10⁻⁷ GBP tex FSR turemers ables sity FR μm μm ple g/g g/g g/g g/g cm³ · s/g Da S g/g · s %ppm % g/100 ml g/s % % Polymer particles without aluminum salt 8 47 6137 21 0 1 73 0.29 1.7 373 5 78 15 0.0 2.0 9 42 55 36 26 3 2 69 0.27 1.2557 6 74 15 0.0 2.4 10 37 48 33 26 6 5 91 0.22 1.2 170 3 80 14 0.1 0.5Polymer particles with aluminum lactate 11 41 54 33 21 1 5 60 0.39 5.1282 4 77 14 0.2 0.9 12 39 53 34 25 1 6 65 0.31 2.6 367 4 79 14 0.5 1.113 36 51 33 25 6 9 74 0.19 1.0 351 4 77 14 0.1 0.6 14 46 60 37 20 0 1 730.30 1.2 510 6 75 13 0.3 0.5 15 45 60 36 22 9 1 65 0.32 1.1 515 8 73 131.0 2.5 16 42 55 36 26 3 2 69 0.27 1.2 557 6 72 12 0.3 0.2 17 39 54 3527 8 5 68 0.32 1.2 510 5 77 14 0.3 1.0 18 28 41 28 24 125 32 86 0.22 0.8565 3 75 14 0.4 0.9 19 25 36 26 23 153 31 111 0.20 0.6 629 5 74 13 1.02.0 Polymer particles with aluminum sulfate 20 32 49 29 22 41 36 70 0.301.2 421 4 79 14 0.4 0.5 21 37 53 33 25 23 17 74 0.31 1.1 373 6 78 13 1.02.6 22 28 43 25 20 93 78 75 0.25 1.2 296 3 80 15 0.3 0.7 23 28 43 27 22106 59 89 0.22 0.9 375 3 81 14 0.1 0.0 24 32 48 30 24 65 35 80 0.29 0.6594 5 75 13 0.3 1.0 25 35 52 32 23 24 25 66 0.30 0.7 684 7 75 13 1.1 2.026 24 36 24 20 275 100 100 0.21 0.6 360 3 80 15 0.2 0.1

TABLE 8 Physical properties of the polymer particles aftersurface-postcrosslinking τ τ τ τ τ Total 0.03 0.1 0.3 0.5 0.7 liquid CRCpsi psi psi psi psi uptake Example g/g s s s s s g Polymer particleswith aluminum salt 8 47.1 464 525 659 832 924 61.5 9 41.7 418 501 546678 716 108.9 10 36.5 324 387 437 571 568 149.5 Polymer particles withaluminum lactate 11 40.8 490 611 493 439 383 73.5 12 38.7 463 563 538489 465 86.2 13 136.0 14 46.4 467 551 598 599 596 54.7 15 46.3 466 551535 535 497 62.9 16 42.6 93.3 17 26.9 261.1 18 27.6 235 281 407 393 391261.0 19 25.3 324.6 Polymer particles with aluminum sulfate 20 31.7134.0 21 37.7 105.8 22 27.5 171.3 23 28.2 272 323 382 436 494 167.8 2432.4 295 358 418 404 363 158.3 25 35.0 309 376 401 389 385 125.8 26 23.6210 258 278 358 338 218.3

Example 27

1200 g of the water-absorbent polymer particles prepared in Example 7b(base polymer) having a content of residual monomers of 4000 ppm wereput into a laboratory ploughshare mixer (model MR5, manufactured byGebrüder Lödige Maschinenbau GmbH, Paderborn, Germany). Asurface-postcrosslinker solution was prepared by mixing 12 g of3-methyl-2-oxazolidinone as described in Table 1 and 60 g of deionizedwater, into a beaker. At a mixer speed of 200 rpm, the aqueous solutionwas sprayed onto the polymer particles within one minute by means of aspray nozzle. The mixing was continued for additional 5 minutes. Theproduct was removed and transferred into another ploughshare mixer(model MR5, manufactured by Gebrüder Lödige Maschinenbau GmbH;Paderborn; Germany) which was heated to 150° C. before. After mixing forfurther 80 minutes at 150° C. with sample taking every 10 minutes, theproduct was removed from the mixer and sifted from 150 to 850 μm. Thesamples were analyzed. The results are summarized in Table 10.

The resulting polymer particles that were surface-postcrosslinked with3-methyl-1,3-oxazolidin-2-one had a bulk density of 70.4 g/100 ml and aflow rate of 11.5 g/s.

Example 28

1200 g of the water-absorbent polymer particles prepared in Example 7b(base polymer) having a content of residual monomers of 4000 ppm wereput into a laboratory ploughshare mixer (model MR5, manufactured byGebrüder Lödige Maschinenbau GmbH; Paderborn; Germany). Asurface-postcrosslinker solution was prepared by mixing 6 g of3-Methyl-3-oxethanmethanol as described in Table 9 and 60 g of deionizedwater, into a beaker. At a mixer speed of 200 rpm, the aqueous solutionwas sprayed onto the polymer particles within one minute by means of aspray nozzle. The mixing was continued for additional 5 minutes. Theproduct was removed and transferred into another ploughshare mixer(model MR5, manufactured by Gebrüder Lödige Maschinenbau GmbH,Paderborn, Germany) which was heated to 150° C. before. After mixing forfurther 80 minutes at 150° C. with sample taking every 10 minutes, theproduct was removed from the mixer and sifted from 150 to 850 μm. Thesamples were analyzed. The results are summarized in Table 10.

The resulting polymer particles that were surface-postcrosslinked with3-methyl-3-oxethanmethanol had a bulk density of 72.2/100 ml and a flowrate of 12.0 g/s.

Example 29

1200 g of the water-absorbent polymer particles prepared in Example 7b(base polymer) having a content of residual monomers of 4000 ppm wereput into a laboratory ploughshare mixer (model MR5, manufactured byGebrüder Lödige Maschinenbau GmbH, Paderborn, Germany). Asurface-postcrosslinker solution was prepared by mixing 6 g of2-oxazolidinone as described in Table 9 and 60 g of deionized water,into a beaker. At a mixer speed of 200 rpm, the aqueous solution wassprayed onto the polymer particles within one minute by means of a spraynozzle. The mixing was continued for additional 5 minutes. The productwas removed and transferred into another ploughshare mixer (model MR5,manufactured by Gebrüder Lödige Maschinenbau GmbH, Paderborn, Germany)which was heated to 150° C. before. After mixing for further 80 minutesat 150° C. with sample taking every 10 minutes, the product was removedfrom the mixer and sifted from 150 to 850 μm. The samples were analyzed.The results are summarized in Table 10.

The resulting polymer particles that were surface-postcrosslinked with1,3-oxazolidin-2-one had a bulk density of 69.7 g/100 ml and a flow rateof 10.8 g/s.

Example 30

1200 g of the water-absorbent polymer particles prepared in Example 7b(base polymer) having a content of residual monomers of 4000 ppm wereput into a laboratory ploughshare mixer (model MR5, manufactured byGebrüder Lödige Maschinenbau GmbH, Paderborn, Germany). Asurface-postcrosslinker solution was prepared by mixing 6 g of Solutionof 3-(2-hydroxyethyl)-2-oxazolidinon and 6 g propandiol as described inTable 9 and 60 g of deionized water, into a beaker. At a mixer speed of200 rpm, the aqueous solution was sprayed onto the polymer particleswithin one minute by means of a spray nozzle. The mixing was continuedfor additional 5 minutes. The product was removed and transferred intoanother ploughshare mixer (model MR5, manufactured by Gebrüder LödigeMaschinenbau GmbH, Paderborn, Germany) which was heated to 165° C.before. After mixing for further 80 minutes at 165° C. with sampletaking every 10 minutes, the product was removed from the mixer andsifted from 150 to 850 μm. The samples were analyzed. The results aresummarized in Table 10.

The resulting polymer particles that were surface-postcrosslinked withof 3-(2-hydroxyethyl)-1,3-oxazolidin-2-one and 6 g propandiol had a bulkdensity of 67.4 g/100 ml and a flow rate of 10.1 g/s.

Example 31

1200 g of the water-absorbent polymer particles prepared in Example 7b(base polymer) having a content of residual monomers of 4000 ppm wereput into a laboratory ploughshare mixer (model MR5, manufactured byGebrüder Lödige Maschinenbau GmbH, Paderborn, Germany). Asurface-postcrosslinker solution was prepared by mixing 3 g ofN,N,N′,N′-Tetrakis(2-hydroxyethyl)adipamide (Primid® XL 552,manufactured by Ems Chemie AG; Domat; Switzerland) as described in Table9 and 60 g of deionized water, into a beaker. At a mixer speed of 200rpm, the aqueous solution was sprayed onto the polymer particles withinone minute by means of a spray nozzle. The mixing was continued foradditional 5 minutes. The product was removed and transferred intoanother ploughshare mixer (model MR5, manufactured by Gebrüder LödigeMaschinenbau GmbH, Paderborn, Germany) which was heated to 160° C.before. After mixing for further 80 minutes at 160° C. with sampletaking every 10 minutes, the product was removed from the mixer andsifted from 150 to 850 μm. The samples were analyzed. The results aresummarized in Table 10.

The resulting polymer particles that were surface-postcrosslinked withN,N,N′,N′-Tetrakis(2-hydroxyethyl)adipamide had a bulk density of 65.8g/100 ml and a flow rate of 10.2 g/s.

Example 32

1200 g of the water-absorbent polymer particles prepared in Example 7b(base polymer) having a content of residual monomers of 4000 ppm wereput into a laboratory ploughshare mixer (model MR5, manufactured byGebrüder Lödige Maschinenbau GmbH, Paderborn, Germany). Asurface-postcrosslinker solution was prepared by mixing 24 g of1.3-Dioxan-2-on as described in Table 9 and 60 g of deionized water,into a beaker. At a mixer speed of 200 rpm, the aqueous solution wassprayed onto the polymer particles within one minute by means of a spraynozzle. The mixing was continued for additional 5 minutes. The productwas removed and transferred into another ploughshare mixer (model MR5,manufactured by Gebrüder Lödige Maschinenbau GmbH, Paderborn, Germany)which was heated to 160° C. before. After mixing for further 80 minutesat 160° C. with sample taking every 10 minutes, the product was removedfrom the mixer and sifted from 150 to 850 μm. The samples were analyzed.The results are summarized in Table 10.

The resulting polymer particles that were surface-postcrosslinked with1.3-Dioxan-2-on had a bulk density of 68.4 g/100 ml and a flow rate of10.5 g/s.

TABLE 9 Formulation of the polymer particles aftersurface-postcrosslinking by using different surface-postcrosslinkersSurface- postcross- Temper- Base Exam- Surface-postcrosslinker - linkerWater ature Time poly- ple Typ bop % bop % ° C. min mer 273-Methyl-2-oxazolidinone 1.00 5.0 150 80 7b 283-Methyl-3-oxethanmethanol 0.50 5.0 150 80 7b 29 2-oxazolidinone 0.505.0 150 80 7b 30 50 wt % solution of 3- 0.50 5.0 165 80 7b(2-hydroxyethyl)-2- oxazolidinone in 1,3- propandiol 31N,N,N′,N′-Tetrakis(2- 0.25 5.0 160 80 7b hydroxyethyl)adipamide 321,3-Dioxan-2-on 2.0 5.0 160 80 7b Bop: based on polymer

TABLE 10 Physical properties of the polymer particles after surface-postcrosslinking by using different surface-postcrosslinkers Total SFCliquid CRC AUNL AUL AUHL 10−7 GBP Vortex uptake Example g/g g/g g/g g/gcm³ · s/g Da s g 27 41.9 56.4 36.1 24.8 0 2 83 114.8 28 41.3 55.0 33.922.8 0 0 61.5 92.5 29 39.3 53.1 33.0 22.1 9 13 54.5 102.2 30 30.9 43.129.2 23.6 0 2 83 208.5 31 34.3 46.3 30.5 24.1 6 8 104 91.9 32 39.2 53.636.6 30.1 12 25 87 110.5

Comparative Examples

AQUA KEEP® SA60SII, AQUA KEEP® SA55XSII, AQUA KEEP® SA60SXII arewater-absorbent polymer particles from SUMITOMO SEIKA CHEMICALS CO.,LTD, produced by a suspension polymerization process.

ASAP® 535, Hysorb® 87075, Hysorb® T9700, Hysorb® B7055, Hysorb® T8760,Hysorb® M7055N, Hysorb® B7015, Hysorb® M7015N, Hysorb® M7015 and Hysorb®7400 are water-absorbent polymer particles from BASF SE, produced by akneader polymerization process.

CE1 and CE2 correspond to water-absorbent polymer particles that areprepared in accordance to Example 7 and Example 26 of WO 2012/045705 A1.

CE3 corresponds to water-absorbent polymer particles that are preparedin accordance to Example 25 of WO 2013/007819 A1.

FIG. 16 is a diagram that shows that the water-absorbent polymerparticles according to the present invention have an improved total quiduptake compared to conventional water-absorbent polymer particles havingthe same centrifuge retention capacity (CRC).

TABLE 11 Physical Properties of Comparison Example τ τ τ τ τ Total SFCExtract- 0.03 0.1 0.3 0.5 0.7 liquid Comparison CRC AUNL AUL AUHL 10⁻⁷GBP Vortex ables FSR psi psi psi psi psi uptake Example g/g g/g g/g g/gcm³ · s/g Da s % g/g · s s s s s s g Comparison Polymer Particles -Suspension Polymerization AQUA 34.4 56 27 14 0 2 38 3.1 108 134 10331667 1534 32.9 KEEP ® SA60SII AQUA 28.9 22 7 6 42 4.4 0.50 82 98 190 549892 KEEP ® SA55XSII AQUA 33.2 15 0 2 38 4.2 0.37 74 138 1215 2103 2154KEEP ® SA60SXII Comparison Polymer Particles - Kneader PolymerizationCE1 25.9 37.0 27.1 22.7 67 34 142 12.9 0.20 195.1 CE2 26.9 42.0 27.422.2 138 90 103 11.9 0.16 295 357 425 437 143.0 CE3 27.6 34.9 26.7 22.898 15 120 12.7 0.38 275 218 271 248 214.2 Hysorb ® 28.8 40.9 29.4 24.345 7 92 8.2 0.25 319 389 467 442 372 155.6 B7075 ASAP ® 30.1 45.6 29.723.5 50 18 13.0 0.18 317 395 425 456 432 161.0 535 Hysorb ® 30.5 46.926.5 19.4 33 55 11.5 0.26 376 406 400 374 372 115.1 T9700 Hysorb ® 29.443.3 29.8 22.3 9 4 93 10.0 0.19 291 363 353 462 106.4 B7055 Hysorb ®30.9 49.2 28.8 19.3 18 33 69 13.7 0.26 300 333 367 483 451 93.1 T8760Hysorb ® 32.0 40.9 29.1 24.5 9 4 97 12.2 0.23 429 469 538 566 702 70.6M7055N Hysorb ® 33.5 45.2 30.3 22.2 4 1 84 9.5 0.21 343 391 365 409 50975.5 B7015 Hysorb ® 34.0 46.3 30.2 22.0 3 2 81 11.9 0.23 260 379 427 4341184 57.0 M7015N Hysorb ® 34.0 47.2 29.5 21.7 2 7 59 12.5 0.29 227 229356 500 74.9 M7015 Hysorb ® 34.8 50.0 28.8 13.2 0 3 35 16.7 0.33 238 276374 738 841 26.0 7400

Example 33

A fluid-absorbent article—the baby diaper of size L—consisting 53% byweight of surface-postcrosslinked polymer of Example 12, wasmanufactured in a standard diaper production process:

The fluid-absorbent article comprises

-   -   (A) an upper liquid-pervious layer comprising a spunbond        nonwoven (three piece cover-stock) having a basis weight of 12        gsm    -   (B) a lower liquid-impervious layer comprising a composite of        breathable polyethylene film and spunbond nonwoven;    -   (C) an absorbent core between (A) and (B) comprising        -   1) lower fluff layer of hydrophilic fibrous matrix of wood            pulp fibers (cellulose fibers) acting as a dusting layer;        -   2) a homogenous mixture of wood pulp fibers (cellulose            fibers) and surface-postcrosslinked polymer. The            fluid-absorbent core holds 53% by weight distributed            surface-postcrosslinked polymer, the quantity of            surface-postcrosslinked polymer within the fluid-absorbent            core is 14.5 g. Dimensions of the fluid-absorbent core:            length: 42 cm; front width: 12.8 cm; crotch width: 8.4 cm;            rear width: 11.8 cm. The density of the fluid-absorbent core            is for the font overall average 0.23 g/cm³, for the insult            zone 0.29 g/cm³, for the back overall average 0.19 g/cm³.            The average thickness of fluid-absorbent core is 0.36 cm.            The fluid-absorbent core is wrapped with a            spunbond-meltblown-spunbond (SMS) nonwoven material having a            basis weight of 10 gsm. The basis weight of fluid-absorbent            core is for the font overall average 990 g/cm³, for the            insult zone 1130 g/cm³, for the back overall average 585            g/cm³.    -   (D) an air through bonded acquisition-distribution layer        between (A) and (C) having a basis weight of 60 g/m²; the        acquisition-distribution layer is rectangular shaped and smaller        than the fluid-absorbent core having a size of about 212 cm².

Dimension of the fluid-absorbent article: length: 51 cm; front width:31.8 cm; crotch width: 22.4 cm; rear width: 31.8 cm. The fluid-absorbentarticle has average weight of 38.1 g.

The fluid-absorbent article further comprises:

-   -   a. flat rubber elastics; elastics from spandex type fibers: 2        leg elastics and 1 cuff elastic    -   b. leg cuffs from synthetic fibers, nonwoven material showing        the layer combination SMS and having a basis weight of between        13 to 15 g/m² and a height of 3.0 cm    -   c. mechanical closure system with landing zone of dimension 16.9        cm×3.4 cm and flexiband closure tapes of 3.1 cm×5.4 cm; attached        to hook fastening tape of 1.9 cm×2.7 cm

The rewet under load and rewet value of the fluid-absorbent article weredetermined. The results are summarized in Table 12 and 13.

Example 34

A fluid-absorbent article—the baby diaper of size L—consisting 49% byweight of surface-postcrosslinked polymer of Example 12 was manufacturedin a standard diaper production process:

The fluid-absorbent article comprises the same components (A), (B) and(D) as in Example 33.

The absorbent core (C) between (A) and (B) comprises

-   -   1) lower fluff layer of hydrophilic fibrous matrix of wood pulp        fibers (cellulose fibers) acting as a dusting layer;    -   2) a homogenous mixture of wood pulp fibers (cellulose fibers)        and surface-postcrosslinked polymer. The fluid-absorbent core        holds 49% by weight distributed surface-postcrosslinked polymer,        the quantity of surface-postcrosslinked polymer within the        fluid-absorbent core is 12.5 g. Dimensions of the        fluid-absorbent core are the same as in Example 32. The density        of the fluid-absorbent core is for the font overall average 0.26        g/cm³, for the insult zone 0.27 g/cm³, for the back overall        average 0.19 g/cm³. The average thickness of fluid-absorbent        core is 0.31 cm. The fluid-absorbent core is wrapped with a        spunbond—meltblown—spunbond (SMS) nonwoven material having a        basis weight of 10 gsm. The basis weight of fluid-absorbent core        is for the font overall average 971 g/cm³, for the insult zone        979 g/cm³, for the back overall average 515 g/cm³.

Dimensions of the fluid-absorbent article are the same as in Example 33.The fluid-absorbent article has average weight of 35.7 g.

The fluid-absorbent article further comprises:

-   -   a. flat rubber elastics as in Example 33    -   b. leg cuffs, as in Example 33    -   c. mechanical closure system, as in Example 33

The rewet under load and rewet value of the fluid-absorbent article weredetermined. The results are summarized in Table 12 and 13.

Example 35

A fluid-absorbent article—the baby diaper of size L—consisting 47.5% byweight of surface-postcrosslinked polymer of Example 12 was manufacturedin a standard diaper production process:

The fluid-absorbent article comprises the same components (A), (B) and(D) as in Example 33.

The absorbent core (C) between (A) and (B) comprises

-   -   1) lower fluff layer of hydrophilic fibrous matrix of wood pulp        fibers (cellulose fibers) acting as a dusting layer;    -   2) a homogenous mixture of wood pulp fibers (cellulose fibers)        and surface-postcrosslinked base polymer. The fluid-absorbent        core holds 47.7% by weight distributed surface-postcrosslinked        polymer, the quantity of surface-postcrosslinked polymer within        the fluid-absorbent core is 11.5 g. Dimensions of the        fluid-absorbent core are the same as in Example 32. The density        of the fluid-absorbent core is for the font overall average 0.24        g/cm³, for the insult zone 0.26 g/cm³, for the back overall        average 0.18 g/cm³. The average thickness of fluid-absorbent        core is 0.32 cm. The fluid-absorbent core is wrapped with a        spunbond—meltblown—spunbond (SMS) nonwoven material having a        basis weight of 10 gsm. The basis weight of fluid-absorbent core        is for the font overall average 928 g/cm³, for the insult zone        967 g/cm³, for the back overall average 495 g/cm³.

Dimensions of the fluid-absorbent article are the same as in Example 33.The fluid-absorbent article has average weight of 34.8 g.

The fluid-absorbent article further comprises:

-   -   a. flat rubber elastics as in Example 33    -   b. leg cuffs, as in Example 33    -   c. mechanical closure system, as in Example 33

The rewet under load and rewet value of the fluid-absorbent article weredetermined. The results are summarized in Table 12 and 13.

Comparative Example

A fluid-absorbent article—the baby diaper of size L—consisting 52% byweight of surface-postcrosslinked polymer HySorb®B7075 (BASF SE,Ludwigshafen, Germany) was manufactured in a standard diaper productionprocess:

The fluid-absorbent article comprises the same components (A), (B) and(D) as in Example 33.

The absorbent core (C) between (A) and (B) comprises

-   -   1) lower fluff layer of hydrophilic fibrous matrix of wood pulp        fibers (cellulose fibers) acting as a dusting layer;    -   2) a homogenous mixture of wood pulp fibers (cellulose fibers)        and surface-postcrosslinked base polymer (HySorb®B7075). The        fluid-absorbent core holds 52% by weight distributed        surface-postcrosslinked polymer, the quantity of        surface-postcrosslinked polymer within the fluid-absorbent core        is 14.5 g. Dimensions of the fluid-absorbent core are the same        as in Example 32. The density of the fluid-absorbent core is for        the font overall average 0.24 g/cm³, for the insult zone 0.25        g/cm³, for the back overall average 0.19 g/cm³. The average        thickness of fluid-absorbent core is 0.34 cm. The        fluid-absorbent core is wrapped with a        spunbond—meltblown—spunbond (SMS) nonwoven material having a        basis weight of 10 gsm. The basis weight of fluid-absorbent core        is for the font overall average 1013 g/cm³, for the insult zone        971 g/cm³, for the back overall average 548 g/cm³.

Dimensions of the fluid-absorbent article are the same as in Example 33.The fluid-absorbent article has average weight of 38 g.

The fluid-absorbent article further comprises:

-   -   a. flat rubber elastics as in Example 33    -   b. leg cuffs, as in Example 33    -   c. mechanical closure system, as in Example 33

The rewet under load and rewet value of the fluid-absorbent article weredetermined. The results are summarized in Table 12 and 13.

TABLE 12 Rewet Under Load Rewet Under Load 1st 3rd 4th 5th Exampleinsult 2nd insult insult insult insult 33 0.10 g 0.12 g 0.10 g 0.09 g0.16 g 34 0.10 g 0.08 g 0.07 g 0.10 g 0.27 g 35 0.10 g 0.06 g 0.07 g0.14 g 0.30 g Comparative Example 0.08 g 0.08 g 0.08 g 0.29 g 0.52 g

TABLE 13 Rewet value Rewet value 1st 3rd 4th 5th Example insult 2ndinsult insult insult insult 33 0.14 g 0.10 g 0.09 g 0.10 g 0.75 g 340.15 g 0.11 g 0.11 g 0.40 g 1.61 g 35 0.14 g 0.10 g 0.09 g 0.87 g 3.84 gComparative Example 0.12 g 0.22 g 0.11 g 0.82 g 4.30 g

The examples demonstrate that the fluid-absorbent article comprisingspherical shaped surface-postcrosslinked polymer particles shows betterrewet performance, even when the loading of spherical shapedsurface-postcrosslinked polymer particles in the absorbent core isreduced up to 20%, in comparison to the fluid-absorbent articlecontaining irregular shaped surface-postcrosslinked polymer particles.

The invention claimed is:
 1. Surface-postcrosslinked water-absorbentpolymer particles having a centrifuge retention capacity from 40 to 55g/g, an absorption under high load from 25 to 35 g/g, a level ofextractable constituents of less than 10% by weight, and a porosity from20 to 40%.
 2. Polymer particles according to claim 1, wherein polymerparticles have a porosity from 25 to 30%.
 3. Surface-postcrosslinkedwater-absorbent polymer particles having a total liquid uptake ofY>−485×ln(X)+1865 wherein Y (g) is the total liquid uptake and X (g/g)is the centrifuge retention capacity, wherein the centrifuge retentioncapacity is at least 25 g/g and the liquid uptake is at least 30 g. 4.Polymer particles according to claim 3, wherein the polymer particleshave a centrifuge retention capacity of at least 40 g/g.
 5. Polymerparticles according to claim 3, wherein the polymer particles have atotal liquid uptake of at least 45 g.
 6. Surface-postcrosslinkedwater-absorbent polymer particles having a change of characteristicswelling time of less than 0.6 and a centrifuge retention capacity of atleast 35 g/g, wherein the change of characteristic swelling time isZ=(τ_(0.5)−τ_(0.1))/τ_(0.5) wherein Z is the change of characteristicswelling time, τ_(0.1) is the characteristic swelling time under apressure of 0.1 psi (6.9 g/cm²) and τ_(0.5) is the characteristicswelling time under a pressure of 0.5 psi (35.0 g/cm²).
 7. Polymerparticles according to claim 6, wherein the polymer particles have achange of characteristic swelling time of less than 0.4.
 8. Polymerparticles according to claim 6, wherein the polymer particles have acentrifuge retention capacity of at least 40 g/g.
 9. Polymer particlesaccording to claim 1, wherein the polymer particles have a meansphericity from 0.80 to 0.95.
 10. Polymer particles according to claim1, wherein the polymer particles have a bulk density from 0.6 to 1g/cm³.
 11. Polymer particles according to claim 1, wherein the polymerparticles have an average particle diameter from 200 to 550 μm.
 12. Afluid-absorbent article, comprising water-absorbent polymer particlesaccording to claim 1 and less than 15% by weight fibrous material and/oradhesives in an absorbent core.